What Are The Filters On The Light Source For Gia Refractometer

11.04: Refractometer

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The refractometer is ane of the virtually of import tools in a gemological laboratory. It indicates (not measures) the refraction index of a gemstone, which often gives vital clues to the identity of a gemstone.

Although ane would expect a refractometer to measure the refraction of light within a gemstone, this is not the example. Instead information technology is based on a unique optical miracle named Full Internal Reflection (or TIR).

For a better understanding of the refractometer, you first need to understand refraction.

Basic

Structure of a gemological refractometer

Effigy \(\PageIndex{1}\): Cantankerous department of a standard gemological refractometer
(modified epitome from an Eickhorst SR 0.005 refractometer)

Low-cal (i) enters through the rear of the refractometer through an opening (1a) in (or earlier) which a yellow sodium filter can exist placed. Information technology then hits a mirror (two) which transmits the calorie-free to the eye of the hemicylinder (three).
This hemicylinder is made of high refractive glass (normally N-LaSF past Schott with a refractive index of ~ 1.88 at n_D and a hardness of about 6.5 on Moh's scale).
At the boundary between the hemicylinder and the gemstone (4), the light will be partially refracted inside the stone and partially reflected in the hemicylinder (see below on Total Internal reflection). The reflected rays (5) will pass through a reading scale (six) and a lens (7) or a series of lenses, depending on the type of refractometer.
The reflected rays hit a mirror (8) which directs the low-cal to the ocular (9) so outside the refractometer to your heart (11).
The ocular (ix) can slide in and out for improve focus and is usually accompanied by a detachable polarizing filter (ten).

As the hemicylinder has a relatively low hardness compared to nearly gemstones, care must exist taken not to scratch it. That would ruin your refractometer, every bit optical contact between the gemstone and the cylinder would be impossible and would give you fake readings.

Total Internal Reflection

Figure \(\PageIndex{ii}\): Inside the refractometer: Total Internal Reflection

When light travels from an optically denser fabric (with higher index of refraction) to an optically rarer cloth (with lower index of refraction), all light that reaches the purlieus of the two materials volition exist either reflected inside the denser material or refracted into the rarer cloth, depending on the angle of incidence of the light.

For every ii media in contact in which lite is traveling from the denser to the rarer medium, the dividing line where either the ray of light is totally reflected or refracted is fixed and can be calculated. This dividing line is named the critical angle (ca). On the left you lot find an prototype showing the critical angle as the ruddy line.
When light reaches the purlieus of the 2 materials at an angle larger than this critical bending (the blueish line), the ray of light will be totally reflected back into the denser textile. Lite reaching the boundary at an bending smaller than the critical angle will exist refracted out of the denser medium (and a small amount will be reflected) into the rarer medium (the greenish line). All low-cal traveling precisely on the critical angle will follow the path of the boundary between the two materials.

Note

In the case above, the light seems to come from three light sources, but the principle is the same when coming from a single point.

In a hemicylinder, the incident and exiting ray always reach the purlieus at a xc-degree angle when directed to the center. Refraction doesn't occur when a light ray is at 90 degrees to the boundary. A hemicylinder is used so in that location volition be no refraction of the light entering or leaving the denser material.

The standard gemological refractometer tin make utilise of this phenomenon considering the reflected rays of lite will appear equally a low-cal expanse on the scale, whilst the refracted rays are not visible (and therefore announced black). The light/night boundary shown on the scale of the refractometer is a visible representation of the critical angle.
The standard gemological refractometer thus measures the disquisitional bending between the glass hemicylinder and the gemstone and plots that on a calibrated scale. This type of refractometer is hence meliorate named a "critical bending refractometer".

Lighting

Proper lighting is 1 of the key features when using the refractometer.

Although ane can get results using a white light source, the standard is monochromatic yellowish light with a wavelength of about 589.3nm. This light source is historically used every bit it was hands produced by called-for salt in a candle (at a very low cost). All gemological refraction indices are based on the use of sodium lite (or n_D). For more data, run across Fraunhofer.

The use of different wavelengths tin produce different readings. Every bit the refractive indices of gemstones are measured with an accuracy of 0.001 decimal, sodium low-cal should be used. All gemological tables of refractive indices are produced using this lite unless otherwise stated.

White lite may be used for single refractive gemstones or to obtain a first impression. One should look for the purlieus betwixt the green and the yellow of the allochromatic white light source.

Nevertheless, for double refractive gemstones, one should then switch to a sodium light source, simply for the reason that the double refraction readings in white low-cal may easily overlap and it would exist impossible to go a correct reading. And of course the boundary between the lighter and darker areas is ameliorate defined, making the reading easier to have.

Always buy a refractometer with either a sodium filter or a sodium light source.

Contact liquids

Here things get a scrap more complicated.

Contact liquids are used to create an optical contact between the hemicylinder and the gemstone. This is to preclude air from trapping between the facet of the rock and the hemicylinder, which would ruin the Total Internal Reflection effect.

As this contact liquid as well has it's ain refractive index, there volition also be Total Internal Reflection between the hemicylinder and the liquid. It is important to ensure that the tiniest drib of liquid is used so the stone doesn't bladder on the liquid. Apply merely plenty to create a "sparse film". Donald Hoover added to this through personal communication that too much liquid will non only elevator upwardly the stone slightly, the reading may also be off slightly due to the refraction inside the liquid (the ray will deviate slightly). With a thin film, this is marginal and will have footling to none event on the reading.

The effect is obviously ii Total Internal Reflection readings, 1 from the hemicylinder-liquid and the other from the liquid-stone boundary (which will be, due to laws of refraction, the same as if no liquid were used). That is the reason you volition likewise see a faint reading nearly the college alphabetize of the scale on the refractometer, which is the reading of the liquid.

The refractive alphabetize of the liquid sets the limit of which stones tin exist tested on the refractometer. Normally, the liquid has a refractive alphabetize of 1.79, but some have a refractive alphabetize of i.81. Y'all can not measure stones that have a RI higher than the liquid used. Stones with a higher RI than the liquid will give yous a "negative reading".

Liquids with higher RI are available, just they are so toxic that they are but used in specially equipped laboratories. They would, of course, as well need a special hemicylinder which volition be of higher RI than the liquid.

You lot should ever shield your contact liquids from lite (especially for the 1.81 type) and intendance should exist taken not to permit the liquids crystallize.

The chemic compositions of the liquids are:

• 1.79 - Saturated solution of sulfur and di-idiomethane
• 1.81 - Saturated solution of sulfur, di-idiomethane and tetraidioethylene

Ever wash your hands after you make physical contact with the liquids -- not only for the smell.

Use of the Refractometer

Video \(\PageIndex{1}\): Video showing how to utilize a refractometer

As with every instrument, success depends on proper usage.

Offset, you apply a very small drib of contact liquid on the center of the hemicylinder of the refractometer, after which y'all place the rock you desire to investigate tabular array down side by side to the hemicylinder. With your fingernail, slide the stone on the center of the hemicylinder. For an oval stone, identify information technology lengthwise.

At this signal, the contact liquid will suck under the facet and provide an optical contact between the rock and the hemicylinder. Practice not apply any pressure to the stone by pushing it downwards on the cylinder equally that would damage the hemicylinder. (Repairs are very costly.) Close the lid of the refractometer to shield the stone from whatever surrounding light. Remove the polarizing filter if it hasn't been removed already.

Now, with the light source in identify at the dorsum, place your best eye (usually your right ane) just before the ocular of the refractometer. You should position your center and then that you look at a direct bending to the ocular, to prevent a "parallax error". The all-time mode to know your eye is in the correct position is if you can see the whole scale (or almost of information technology) without moving your eye.

Now find the dividing line between light and night on the scale. (For gemstones cut en-cabochon, the technique is slightly different. Meet the "distant vision" method below.) If the scale seems blurry, you can slide the ocular in and out for meliorate focus. Now you tin can start taking your readings (explained below).

When y'all are finished, gently slide the rock off the hemicylinder and remove the stone with your fingers if possible. It is important to keep the hemicylinder make clean, and so use a make clean fabric or tissue to gently wipe whatever remaining contact liquid from the cylinder. Do this gently without any pressure, making a North-Southward motion.

As mentioned above, the hemicylinder is made of a relatively low hardness glass and can easily scratch. Then always brand sure you keep annoying materials and sharp objects (like tweezers) away from the hemicylinder.

Wait at the images below to see how to properly use the refractometer.

Click images to enlarge

Figure \(\PageIndex{3}\): Open liquid canteen and get small-scale drop

Figure \(\PageIndex{iv}\): Advisedly place on middle of hemicylinder

Figure \(\PageIndex{5}\): Drop should be no larger than this!

Figure \(\PageIndex{half-dozen}\): Place stone parallel to length of hemicylinder

N.B: Some people find it hard to get a small drop of liquid directly from the bottle. A different technique is to place a serial of modest drops (commonly 2 or 3) adjacent to the hemicylinder and place the stone on the smallest drib, so slide the stone and liquid together onto the hemicylinder. Alternatively, one can lose excess liquid from the liquid rod by making a few drops next to the hemicylinder and then use the remainder directly onto the refractometer'due south hemicylinder. Whichever method one prefers will piece of work.

Effigy \(\PageIndex{7}\): 1.544

We notate refractometer readings to a precision of 0.001 (one thousandths). The refractometer scale has subdivision indicators to 0.01 (one hundredths). Between the two horizontal bars which indicate the 0.01, you will need to estimate the final precision.

In the paradigm on the right, yous volition come across that the shadow edge is between the 1.54 and the 1.55 bars. Between these ii values, we need to discover the last precision. As it is just above the middle, the last precision is 0.004. Then the reading is 1.544.

Estimating the last decimal needs some do. Some refractometers, like the Eickhorst ones, have a more detailed sectionalization of the scales which makes taking a reading easier. With a fiddling experience, you will discover an easier-to-read scale is not needed.

Faceted gemstones

Following is the method for taking RI readings that is used for faceted gemstones. En-cabochon and sphere cut gemstones require a somewhat dissimilar technique which is explained in the "distant vision" section.

Effigy \(\PageIndex{8}\): Starting position 1st reading

Effigy \(\PageIndex{ix}\): 45-caste rotation 2nd reading

Figure \(\PageIndex{10}\): xc-degree rotation 3rd reading

Figure \(\PageIndex{xi}\): 135-caste rotation 4th reading

When taking refractometer readings, ane usually starts with the largest facet (which is commonly the tabular array facet). Identify your stone in the starting position, then close the hat of the refractometer. Make sure the calorie-free source is on.

Position your eye in forepart of the ocular in a way so that it is at a straight angle with the refractometer calibration. Y'all will now most likely see a dark region at the top of the scale and a lighter region in the lower office. If you take chosen a monochromatic sodium light source, at that place volition exist a sharp line betwixt the lighter and darker areas. That line is named the "shadow edge". (You may also notice ii less abrupt "shadow edges".)

Place the polarization filter on the ocular and, while looking at the scale, turn the polarizer 90 degrees left and correct. You lot volition detect either of two possibilities:

1. only one shadow edge is seen
   • the stone is either isotropic or
   • the incident calorie-free reaches the stone at an angle parallel to the optic axis and y'all should turn the stone 90 degrees
2. you run into the shadow edge move betwixt 2 values on the scale
   • the rock is uniaxial or
   • the stone is biaxial

• In the outset case, where only one shadow border is seen, the reading for the shadow edge will remain constant during a 135-caste rotation of the stone. For every rotation reading, take two measurements: i with the polarizing filter in North-S position and i with the polarizing filter in Due east-West position.

The readings in the images below indicate a unmarried refractive (isotropic) stone with RI = 1.527, which is almost likely drinking glass. (If one finds a single refractive transparent faceted stone with an RI betwixt 1.l and 1.70, it is nigh likely glass). Taking iv sets of readings (with the polarizer in both positions) on a single refractive stone looks like overkill, which information technology is; take them anyway.

First reading		Second reading		Third reading		Fourth reading
1.527	1.527	one.527	ane.527	1.527	1.527	i.527	1.527

• In the 2d case, where the shadow edge moves between ii values on the scale, write down both values you see, in table form below each other.

Below are 4 sets of readings of a double refractive stone with a uniaxial optic graphic symbol (where one reading value remains abiding). For every prepare of readings, y'all rotate the stone 45 degrees with your fingers without applying pressure while leaving the stone in contact with the hemicylinder.

Beginning reading		Second reading		3rd reading		Quaternary reading
1.544 ω	1.553 ε	1.544 ω	i.552 ε	1.544 ω	1.549 ε	1.544 ω	ane.552 ε

	1st	2nd	third	4th
lower readings ω	1.544	one.544	1.544	1.544
higher readings ε	1.553	i.552	1.549	1.552

While taking your refractometer readings, write down the values y'all read on the scale. For every set of readings, the polarization filter is turned 90 degrees. In add-on to this, you tin also have a fifth reading (180-caste rotation).

In the example above, the lower readings (1.544) stay abiding while the higher readings vary. In other gemstones, the higher value may remain constant while the lower value changes.

Annotation

The lower reading is the reading of lower value, not lower on the scale.

The RI of this stone is 1.544 - 1.553 (smallest lower reading and largest higher reading). This indicates quartz.

To calculate the birefringence of the gemstone being tested, you take the maximum difference between the largest higher reading and the smallest lower reading. In this example, that is 1.553 - ane.544 = 0.009 .

Some gemstones have a lower reading that falls within the range of the refractometer (and the liquid), while the higher reading falls outside the range. Those gemstones volition give you just i reading on the refractometer and should not be dislocated with isotropic gemstones.

• Gemstones may likewise have two variable lower and higher readings, but the process remains the same. You lot write down the lower and higher readings in a table and calculate the birefringence.

Get-go reading		2d reading		Third reading		Fourth reading
1.613	1.619	ane.611 α	ane.616	1.614	ane.619	one.611 α	1.620 γ

These readings give a biaxial reading with RI = 1.611-1.620 and a birefringence of 0.009, indicating topaz.

	1st	2nd	3rd	4th	departure
lower readings	one.613	ane.611	ane.614	one.611	0.003
higher readings	1.619	1.616	ane.619	i.620	0.004

Yous may have noticed some odd looking letters in the image footers, like α, γ, ε, and ω (and β which will be seen later on). They are not typos but Greek letters whose meanings will become apparent in the discussion on optical sign. You lot will likewise learn why nosotros added the "difference" in the biaxial tabular array.

Optical character

Optical character refers to how rays of low-cal travel in gemstones (or virtually other materials).
In uniaxial and biaxial materials, the incoming light will exist polarized in ii (uniaxial) or three (biaxial) vibrational directions which all travel at different speeds inside the gemstone. This is due to the molecular packing inside the stone. For a ameliorate agreement, nosotros refer to the word on double refraction.

Gemstones are divided into three categories (characters) depending on the way a ray of light behaves as it passes through the stone:

1. isotropic
2. uniaxial
3. biaxial

• Isotropic stones are stones in which light travels in all directions at equal speed.

Among those stones are the ones that class in the cubic system too as baggy stones, like glass.

• On the refractometer, you will come across one constant reading.

• Uniaxial means that light travels differently in ii directions.

One ray of light volition vibrate in the horizontal plane, which we call the ordinary ray (ω). The other will vibrate in a vertical plane along the c-axis and is chosen the extra-ordinary ray (ε). This actress-ordinary ray is also the optic axis (the axis forth which light behaves as if being isotropic).

Gemstones that are uniaxial by nature belong to the tetragonal, hexagonal and trigonal crystal systems.

• You will see one abiding and ane variable reading on the refractometer.

• Biaxial gemstones split upward incoming light into two rays besides; however, the crystallographic directions are labeled as the α, γ, and β rays. The two rays both human action as actress-ordinary rays.

Stones with a biaxial optic character have two optic axes.

The orthorhombic, monoclinic and triclinic crystal systems are biaxial.

• This will be shown by two variable readings on the refractometer.

Spot readings (afar vision method)

This is the method used to judge the RI of en-cabochon cutting gemstones.

You place a very small drop of contact liquid on the hemicylinder and place the stone on the drop, on it'south most convex side (equally in the prototype beneath). Remove the polarization filter (if not already washed) and close the lid.

Figure \(\PageIndex{12}\)

Move your head back almost 30 cm from the ocular and look direct to the calibration. On the scale, you lot'll encounter a reflection of the contact liquid droplet. When yous move your head slightly in a "yes-motion", you'll find the droplet move over the scale. Try to fixate the bespeak where half of the droplet is nighttime and the other half is bright.

Effigy \(\PageIndex{xiii}\)

The epitome above shows three stages while moving your head. The top droplet is also light and the bottom one is too dark. The one in the middle shows a good one-half nighttime/one-half bright droplet.

Now motion your head toward the ocular and estimate the Refractive Alphabetize. Different with faceted gemstones, we estimate to a 0.01 precision when using this method. The prototype below shows the reflection of the liquid which is one-half bright/half dark at 1.54. This gemstone may be Amber.

Figure \(\PageIndex{14}\)

Alas, i cannot determine birefringence using this method, unless the birefringence is quite large (equally with the carbonates). The "birefringence blink" or "carbonate blink" technique makes use of a larger drop of contact liquid and a polarizing plate. As the plate is rotated, the spot will exist seen to blink. A crude estimation of birefringence can be made by this technique.

Avant-garde

Optical sign

Optic sign in birefringent gemstones is shown as either a plus (+) or a minus (-). The reasons why some stone have a positive sign and others a negative sign lies in the orientation of molecules inside the gemstone. This is explained by the apply of an indicatrix in the refraction department.

Isotropic gemstones do not have an optical sign. Light travels at the same speed in all directions.

Uniaxial stones may have either a positive (+) optical sign or a negative (-) one.
We summate the optic sign by deducting the ordinary ray (ω) from the extra-ordinary ray (ε). So in the example of Quartz with ε = ane.553 and ω = i.544 that will give us a positive number of 0.009. Hence the optical sign is positive.
A full refractometer result for quartz will therefore be "RI = 1.553-one.544 uniaxial +" and a birefringence of 0.009.

In uniaxial gemstones, the abiding reading is always the ordinary ray (ω).

If the ordinary ray is the higher reading in a gemstone (every bit in the case of Scapolite), there volition be a negative optical sign. For instance if you take the following readings: ε = 1.549 and ω = ane.560, the calculation will be i.549 - 1.560 = -0.011 (so a negative).
This is how we separate Quartz from Scapolite nearly of the time, the starting time is uniaxial +, the latter is uniaxial -.

Biaxial gemstones tin also be either positive or negative for the same reasons; yet, biaxial minerals accept 3 values that stand for with the crystallographic axes. These are the α (Greek letter alpha), β (Greek letter beta) and γ (Greek letter gamma).
The indicatrix of biaxial materials is somewhat more circuitous than the uniaxial one.

In do, we are not concerned with the intermediate β value, merely with the higher and lower readings we find on the refractometer. As shown previous, we take 4 sets of readings for every orientation of the rock (0 degrees, 45 degrees, ninety degrees, and 135 degrees). If nosotros put the readings in a nice table, we can summate whether the higher or the lower readings vary the well-nigh.

	1st	2d	3rd	4th	difference
lower readings α	1.613	1.611	1.614	1.611	0.003
higher readings γ	i.619	1.616	1.619	ane.620	0.004

Equally can be seen in the tabular array on the correct, the college readings vary the most (0.004) opposed to the lower readings (0.003), this indicates a positive sign. If the lower reading would accept varied the near it would have been biaxial negative.
So for this Topaz, the full reading would exist: "RI= 1.611-1.620 biaxial +" of course nosotros likewise mention the birefringence every bit "DR = 0.009".

Equally a word of caution, the caption higher up is a rough method equally the β value has not been determined. When at that place is doubt most the identity of the gemstone due to the optic sign, make certain you decide the true value of β (here it could be either 1.614 or i.616). When the polarizer is used properly, one will observe that true β is at 1.614 for this stone.

Overview of the crystal systems

Construction	Structure blazon Crystal axes Angles	Symmetry (of highest crystal class)	Optic character	Refractive alphabetize (RI)	Optic sign	Pleochroism	Jewel examples
Baggy	No order No axes	No symmetry	Isotropic Singly refractive	1 RI n	None	None	Glass Amber
Cubic	Isometric: 1 axis length ai = a2 = a3 All at 90°	13 planes 9 axes Centre	Isotropic Singly refractive	1 RI due north	None	None	Diamond Spinel Garnet
Tetragonal	Dimetric: 2 axis lengths a1 = a2 ≠ c All at 90°	5 planes 5 axes Center	Anisotropic Doubly refractive Uniaxial	two RIs nw and ne	+ = due northe > nwest – = northwarde < nw	May exist dichroic	Zircon
Hexagonal	Dimetric: ii centrality lengths a1 = a2 = a3 ≠ c a axes at 60°; c axis at 90° to their airplane	7 planes 7 axes Center	Anisotropic Doubly refractive Uniaxial	2 RIs northwardwestward and ne	+ = neastward > nwestward – = neastward < nw	May exist dichroic	Beryl Apatite
Trigonal	Dimetric: 2 centrality lengths a1 = aii = a3 ≠ c a axes at 60°; c centrality at 90° to their airplane	three planes four axes Center	Anisotropic Doubly refractive Uniaxial	two RIs due northw and ne	+ = due northdue east > nw – = northwarddue east < nw	May be dichroic	Corundum Quartz Tourmaline
Orthorhombic	Trimetric: 3 centrality lengths a ≠ b ≠ c All at 90°	three planes three axes Center	Anisotropic Doubly refractive Biaxial	iii RIs na, nb, nk	+ = northb closer to due northa – = nb closer to due northgrand ± = nb midway between na & ng	May exist trichroic	Topaz Zoisite Olivine (peridot)
Monoclinic	Trimetric: 3 axis lengths a ≠ b ≠ c 2 axes at 90°; i axis oblique	ane centrality i aeroplane Center	Anisotropic Doubly refractive Biaxial	three RIs na, nb, northwardg	+ = nb closer to due northa – = nb closer to nthou ± = due northb midway betwixt northwarda & northwardgrand	May be trichroic	Orthoclase Spodumene
Triclinic	Trimetric: 3 centrality lengths a ≠ b ≠ c all axes oblique	No planes No axes Middle	Anisotropic Doubly refractive Biaxial	three RIs na, nb, due northchiliad	+ = due northb closer to northwarda – = nb closer to north1000 ± = nb midway between na & northwardg	May exist trichroic	Axinite Labradorite

Optic character/sign with the Refractometer

Optic graphic symbol/bend variations: Uniaxial or biaxial

1. Two abiding curves = Uniaxial

2. 2 variable curves = Biaxial

three. One abiding/1 variable which meet = Uniaxial

4. One abiding/ane variable which don't come across:

Check the polaroid angle of the constant curve

a. Biaxial = polaroid angle of constant bend = 90°

b. Uniaxial = polaroid angle of abiding bend ≠ 90°

Optic sign

Uniaxial stones

1. High RI curve varies = (+)

2. Low RI curve varies = (-)

3. Both curves constant: At 0° polaroid angle, only the o-ray is seen

a. If depression curve is seen = (+)

a. If high curve is seen = (-)

Biaxial stones

1. If n_b is closer to due north_a, the precious stone is (+)

2. If n_b is closer to n_g, the gem is (-)

three. If due north_b is halfway between n_a and n_g, the gem is (±)

4. If two possible betas exist, false beta volition have a polaroid angle equal to ninety°. True beta volition accept a polaroid angle unequal to 90°.

Polaroid angle

• 0° polaroid angle is when the polarization axis of light transmitted through the plate is parallel to the refractometer calibration divisions.
• xc° polaroid bending is when the polarization centrality of low-cal transmitted through the plate is perpendicular to the refractometer scale divisions.

Symbols

Uniaxial crystals

• n_w = omega, the constant RI of a uniaxial crystal
• n_e = epsilon, the variable RI of a uniaxial crystal

Biaxial crystals

• north_a = alpha, the lowest RI of a biaxial crystal
• n_b = beta, the intermediate RI of a biaxial crystal
• n_grand = gamma, the highest RI of a biaxial crystal

Vivid line technique

In some cases, you may find information technology very hard to become a articulate boundary between light and nighttime using conventional refractometer techniques. In those rare cases you lot may find information technology useful to illuminate from the superlative of the hemicylinder instead of from beneath.

Cover up the illumination opening at the rear of the refractometer and open the hat. Identify the stone in position every bit usual and illuminate the rock/hemicylinder in a way that the lite is grazing over the surface of the hemicylinder.
This volition give yous a very bright area when you look through the ocular and/or a very bright line showing the RI value. This technique is best carried out in a night environs with a lite source that is pointed from the back of the stone (in the management of the observer). The junction of the rock'due south facets should exist perpendicular to the length axis of the hemicylinder.
With some practice, this volition give you a 0.001 precision.

When allochromatic white lite is used, 1 can make up one's mind the relative dispersion of the gemstone too equally absorption lines in some cases.

Kerez issue

Some green tourmalines may show up to viii shadow edges (tourmaline is uniaxial and should only show two shadow edges in one reading). This is to current noesis due to estrus and/or thermal shock while polishing the table facets.
Little documentation on this bailiwick is at hand.

Peter Read added the following in personal correspondence:
"The issue in green tourmaline was start reported in 1967 by R. K. Mitchell [ed.: Journal of Gemmology Vol. 10, 194 (1967)] and the proper noun 'Kerez consequence' was suggested by him. Work on the event has since been carried out past Schiffmann and Prof. H. Banking company. In GEMS, the effect first appeared in the 5th edition and was inserted in Chapter half dozen (Topaz & Tourmaline) by the late Robert Kammerling one-time Director of Identification & Research, GIA Jewel Trade Laboratory, USA. I empathize that the issue is mainly caused by thermal daze due to polishing, and not to chemical constituents."

Dietrich [1985] mentions that the highest of these readings (lowest on the scale) are the correct ones.

This phenomenon was named after C.J. Kerez.

Different types of refractometers

A word of caution to all neophyte gemologists on buying a refractometer. Nowadays cheap refractometers are offered on the internet for as low every bit USD 100.00. They are generally made in China and one shouldn't expect too much from them. Especially obtaining an RI for pocket-size and en-cabochon cut stones may prove to be difficult.
Some sellers put their own respected visitor logo on them and pass them on as the best your money can buy.
Always test your new refractometer with a small stone with a known refractive index and make sure it is precise at 0.001.

Although the toll is very tempting, a good refractometer is more than costly but volition last a lifetime when handled with intendance.
Some of them are outlined below.

The GemPro refractometer

GemPro refractometers are straight view type refractometers just like the duplex Ii that GIA makes. Straight view refractometers accept removable eyepiece lenses that enable spot reading of cabochons. Other type refractometers tin't do this well because they take a different prism design. The eyepiece used with the GemPro refractometer is a special achromatic lens that gives excellent resolution when birefringence and other readings are being observed. The hemicylinders are fabricated of a special German drinking glass made by Schott glass company. These hemicylinders are tough to scratch and resistant to chemicals. Tarnish from the air does non happen with this type of glass. Supplied with monochromatic filter, RI liquid, and MagLight.

The Rayner Dialdex refractometer

This refractometer differs from near TIR refractometers that it doesn't have an internal scale to read the values from. Instead, yous will see a "window" with a bright area. By turning a "wheel" on the side of the refractometer, a vertical black band will appear which should be lined up with the lower border of the brilliant expanse. Afterward this 1 takes the reading from the calibrated wheel.
An external light source should exist used.

The Duplex refractometer

Fabricated in the Usa, this refractometer has an actress big window of view. Making information technology easier to find shadows.
No built-in low-cal source, an external one should be used.

The Eickhorst refractometer

In contrast to virtually refractometers, the Eickhorst refractometers have a calibrated scale with 0.005 precision (opposed to the usual 0.01) and this makes estimating the tertiary decimal easier.
Eickhorst also offers gemology modules of bully quality and appealing advent. Some models take an internal lite source.

The Topcon refractometer

This refractometer is made in Japan. Very sturdy metal case and made to last. Information technology is i of the most expensive refractometers on the market.
No internal light source.

The Kruess refractometer

Kruess is a long-established German manufacturer of all sorts of refractometers (not merely for gemological purposes). Their line in excellent gemological refractometers includes portable and standard ones, with or without congenital-in lightning.

Refractive Index of Common Jewel Minerals

Some of the values listed below reflect values which are farthermost possibilities for the gem.
In other words, highs and lows which are, merely rarely, seen.
Remember to always check values for birefringence, as it can be as diagnostic as RI.

Precious stone Mineral	Refractive Index	Birefringence
Actinolite	1.614 - 1.655	0.022 - 0.026
Adventurine (Quartz)	ane.544 - 1.553	0.009
Agate	1.535 - 1.539	0.004
Air (as a betoken of interest)	1.0003
Albite (Feldspar)	1.527 - 1.538	0.011
Alexandrite	one.745 - 1.759	0.009 - 0.010
Allanite	1.640 - ane.828	0.013 - 0.036
Almandine (Garnet)	1.775 - one.830
Amazonite (Feldspar)	1.514 - 1.539	0.008 - 0.010
Bister	1.539 - one.545
Amblygonite	1.578 - 1.612	0.020 - 0.021
Amethyst	1.544 - one.533	0.009
Ametrine	1.544 - 1.553	0.009
Anatase	two.488 - 2.564	0.046 - 0.067
Andalusite	one.627 - ane.650	0.007 - 0.011
Andesine (Feldspar)	1.543 - 1.551	0.008
Andradite (Garnet)	1.880 - i.940
Angelsite	one.877 - 1.894	0.017
Anorthite (Feldspar)	1.577 - ane.590	0.013
Apatite	ane.628 - 1.650	0.001 - 0.013
Apophyllite	1.530 - 1.540	0.001 or less
Aquamarine (Beryl)	1.567 - 1.590	0.005 - 0.007
Aragonite	1.530 - 1.685	0.155
Augelite	1.574 - one.588	0.014 - 0.020
Axinite	1.672 - 1.694	0.010 - 0.012
Azurite	1.720 - 1.850	0.110
Barite	1.636 - 1.648	0.012
Bastnäsite	1.717 - ane.818
Benitoite	1.757 - 1.804	0.047
Beryl	ane.563 - i.620	0.004 - 0.009
Beryllonite	1.552 - 1.562	0.009
Bixbite (Beryl)	1.568 - ane.572	0.004 - 0.008
Boracite	1.658 - 1.673	0.024
Brazilianite	1.602 - 1.625	0.019 - 0.021
Bronzite	1.665 - 1.703	0.015
Bytownite (Feldspar)	1.561 - i.570	0.009
Calcite	1.486 - 1.740	0.172 - 0.190
Carnelian	i.535 - 1.539	0.004
Cassiterite	1.995 - ii.095	0.098
Celestite	i.619 - ane.635	0.009 - 0.012
Cerussite	i.803 - 2.078	0.274
Chalcedony	one.535 - ane.539	0.004
Chrome Diopside	1.668 - 1.702	0.028
Chrysoberyl	ane.740 - 1.777	0.008 - 0.012
Chrysocolla	ane.575 - ane.635	0.023 - 0.040
Chrysoprase	1.535 - 1.539	0.004
Citrine	one.544 - one.553	0.009
Clinozoisite	1.670 - one.734	0.028 - 0.041
Colemanite	1.586 - one.614	0.028
Coral	1.550 - 1.580	0.160
Crocoite	ii.290 - ii.660	0.270
Cubic Zirconia	2.170
Cuprite	2.848
Danburite	1.627 - 1.639	0.006 - 0.008
Datolite	1.621 - i.675	0.044 - 0.047
Demantoid (Andradite)	1.880 - 1.888
Diamond	2.417
Diopside	i.664 - 1.721	0.024 - 0.031
Dioptase	one.645 - i.720	0.053
Dolomite	1.500 - one.703	0.179 - 0.185
Dumortierite	ane.668 - 1.723	0.150 - 0.370
Ekanite	i.590 - 1.596	0.001
Emerald (Beryl)	1.575 - 1.602	0.004 - 0.009
Emerald (synth. flux)	1.553 - i.580	0.003 - 0.005
Emerald (synth. hydro)	one.563 - 1.620	0.003 - 0.008
Enstatite	ane.650 - 1.680	0.010
Epidote	1.715 - 1.797	0.015 - 0.049
Euclase	1.650 - 1.677	0.019 - 0.025
Fayalite (Olivine)	ane.827 - 1.879	0.052
Fluorite	1.432 - 1.434
Friedelite	1.625 - one.664
Gahnite	1.790 - 1.820 (isometric)
Gahnospinel	1.735 - ane.790
Genthelvite	1.742 - 1.745
Glass (man-made)	1.520 - 1.550
Golden	0.470
Goshenite (Beryl)	ane.566 - 1.602	0.004 - 0.008
Grossular (Garnet)	i.730 - 1.760
Hackmanite	i.483 - i.487
Hambergite	one.550 - 1.630	0.072
Hauyne	i.496 - 1.505
Heliodor (Beryl)	i.566 - 1.579	0.005 - 0.009
Hematite	2.880 - three.220	0.280
Hemimorphite	1.614 - 1.636	0.022
Hessonite (Garnet)	1.742 - ane.748
Hiddenite (Spodumene)	1.653 - 1.682	0.014 - 0.027
Howlite	ane.583 - 1.608	0.022
Hydrogrossular (Garnet)	1.690 - 1.730
Hypersthene	i.686 - 1.772	0.017
Idocrase	1.655 - 1.761	0.003 - 0.018
Iolite	1.533 - ane.596	0.005 - 0.018
Ivory	1.535 - 1.555
Jadeite	1.640 - ane.667	0.012 - 0.020
Jasper (Quartz)	1.544 - ane.553
Kornerupine	one.665 - one.700	0.013 - 0.017
Kunzite (Spodumene)	1.653 - 1.682	0.014 - 0.027
Kyanite	i.710 - ane.735	0.017
Labradorite (Feldspar)	1.560 - 1.572	0.012
Lapis Lazuli	i.500
Lazulite	1.604 - i.662	0.031 - 0.036
Leucite	1.504 - 1.510
Magnesite	i.509 - ane.717	0.022
Malachite	1.655 - i.909	0.254
Maw-Sit down-Sit	ane.520 - 1.680
Microline (Feldspar)	1.514 - i.539	0.008 - 0.010
Moissanite	2.648 - 2.691	0.043
Moldavite	1.460 - one.540
Moonstone (Feldspar)	i.518 - i.526	0.005 - 0.008
Morganite (Beryl)	1.572 - 1.600	0.008 - 0.009
Natrolite	1.473 - i.496	0.012
Nephrite	ane.600 - 1.640	0.027
Obsidian	1.450 - one.520
Oligoclase (Feldspar)	1.542 - i.549	0.007
Onyx	1.535 - 1.539	0.004
Opal	1.370 - 1.470
Orthoclase (Feldspar)	1.518 - i.539	0.005 - 0.008
Painite	ane.787 - 1.816	0.027 - 0.028
Pearl	1.530 - i.685	0.155
Pectolite	1.595 - 1.645	0.036
Periclase	1.736
Peridot (Olivine)	1.650 - 1.681	0.033 - 0.038
Petalite	i.502 - 1.520	0.012 - 0.014
Phenakite	1.650 - 1.695	0.016
Phosphophyllite	ane.595 - 1.621	0.021 - 0.033
Prasiolite (Quartz)	ane.544 - 1.553	0.009
Prehnite	1.611 - 1.665	0.021 - 0.033
Proustite	2.792 - iii.088	0.296
Purpurite	one.850 - i.920	0.007
Pyrope (Garnet)	1.730 - one.766
Quartz	1.544 - i.553	0.009
Rhodizite	1.694
Rhodochrosite	1.578 - 1.840	0.201 - 0.220
Rhodolite (Garnet)	1.745 - 1.760
Rhodonite	1.711 - 1.752	0.011 - 0.014
Blood-red (Corundum)	1.762-1.770	0.008 - 0.009
Rutile	2.620 - 2.900	0.287
Sanidine (Feldspar)	ane.518 - 1.534	0.005 - 0.008
Sapphire (Corundum)	i.762-1.770	0.008 - 0.009
Sapphirine	one.714 - 1.723	0.006
Scapolite	1.536 - 1.596	0.015 - 0.026
Scheelite	i.918 - 1.936	0.016
Serpentine	i.490 - ane.575	0.014
Shattuckite	1.752 - 1.815	0.063
Siderite	one.633 - 1.873	0.240
Sillimanite	i.654 - 1.683	0.020
Silver	0.180
Sinhalite	1.665 - 1.712	0.035 - 0.037
Smithsonite	i.620 - 1.850	0.227
Sodalite	i.483 - 1.487
Spessartine (Garnet)	1.790 - 1.810
Sphalerite	2.400
Sphene	one.900 - ii.034	0.100 - 0.192
Spinel	i.712 - 1.735 (isometric)
Spinel (syn. flame fushion)	ane.710 - one.740 (isometric)
Spodumene	one.653 - ane.682	0.014 - 0.027
Staurolite	1.736 - 1.762	0.011 - 0.015
Strontium Titanate	2.400
Taaffeite	1.717 - ane.730	0.004 - 0.009
Tantalite	2.260 - 2.430	0.160
Tanzanite (Zoisite)	1.692 - i.705	0.009
Tektite	1.460 - 1.540
Thomsonite	ane.497 - one.544	0.021
Thulite (Zoisite)	i.692 - 1.705	0.006
Tiger middle (Quartz)	1.544 - one.553	0.009
Topaz	1.609 - 1.643	0.008 - 0.011
Tourmaline	1.620 and 1.640 (normally)	0.020
Tremolite	i.560 - 1.643	0.017 - 0.027
Tsavorite (Garnet)	1.560 - 1.643 (isometric)
Tugtupite	1.494 - 1.504	0.006 - 0.008
Turquoise	ane.610 - 1.650	0.040
Ulexite	one.496 - i.519	0.023
Uvarovite (Garnet)	1.740 - i.870 (isometric)
Vanadinite	2.350 - 2.416	0.066
Variscite	one.560 - 1.594	0.031
Vesuvianite	one.655 - 1.761	0.003 - 0.018
Vivianite	1.569 - 1.675	0.040 - 0.059
Water (at 20°C)	1.3328
Willemite	i.690 - one.723	0.028
Wulfenite	ii.280 - 2.405	0.122
Zincite	ii.013 - ii.029	0.016
Zircon, High	one.970 - 2.025	0.000 - 0.008
Zircon, Medium	one.840 - ane.970	0.008 - 0.043
Zircon, Low	ane.780 - one.850	0.036 - 0.059
Zoisite	one.685 - i.725	0.004 - 0.008

Sources

• Gemmology 3rd edition (2005) - Peter Read
• Gemology - C.South. Hurlbut and G.Southward.Switzer (1981) Gemology. New York, The states., Wiley, 1st ed., 243 pp.
• Gems, Their Sources, Descriptions and Identification 4th edition - Robert Webster, Anderson
• Gem Identification Fabricated Piece of cake 3th edition - Bonanno, Antoinette Matlins
• Precious stone-A Foundation and Diploma notes
• Refraction Anomalies in Tourmalines - R. Keith Mitchell, Journal of Gemmology Vol. 10, 194 (1967)
• Better refractometer results with the Brilliant Line technique - Dr D.B. Hoover and C. Williams, Journal of Gemmology Vol. thirty No. v/half dozen, 287-297 (2007)
• The Tourmaline Grouping (1985) - Richard Dietrich ISBN 0442218575

What Are The Filters On The Light Source For Gia Refractometer,

Source: https://geo.libretexts.org/Bookshelves/Geology/Book%3A_Gemology/11%3A_Equipment_used_to_Identify_Gemstones/11.04%3A_Refractometer

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