GO 340 Gemstones & Gemology
ES 567 Gemstones of the World
Dr. Susan Ward Aber, Geologist & Gemologist
Emporia State University
Emporia, Kansas USA


Gem Testing

Visual and optical properties include those properties dependent upon visible light and are observable with the unaided eye. The gems optical and physical properties may be determined using visual observations combined with some instrumentation, which leads to gem identification. Some of the basics are explained below.

UV LW/SW Light Color Filter Refractometer
Polariscope Dichroscope Microscope
10X Triplet Loupe

UV LW/SW Light for Testing Luminescence/Fluorescence

Gems that emit visible light after exposure to short-wave or long-wave ultraviolet radiation, they are said to be luminescent, or more specifically, flourescent. A gem is phosphorescent if the luminescence continues after the UV light source has been removed. This phenomenon of fluorescence and/or phosphorescence is the result of UV radiation absorbed by impurities or structural defects within the crystal structure, resulting in an oscillation of electrons between energy levels, and transmission of visible light.

Photo date 1/2000; © by S.W. Aber

Fluorescence produces vivid colors when an ultraviolet or invisible light source is directed at a gemstone. Where did this term come from? In the middle of the nineteenth century, Professor George Stokes was experimenting with sunlight and the mineral fluorite. He was in a darkened room and admitted sunlight through a small hole in a window shutter. When sunlight fell on a colorless fluorite specimen, he noticed the mineral displayed a bright blue color. Stokes coined the term fluorescence to describe the phenomenon he observed from the fluor-spar or fluorite, which he said was analogous to opalescence observed in opals (Robbins, 1994, p. 63). Therefore, fluorite is the basis for the word fluorescence but not all fluorite fluoresces! Try saying that quickly, three times in a row.

The fluorescence can be a bold green, orangish-red, or whitish blue, and vary in intensity. Fluorescence may be unpredictable because some gems will have no reaction to the UV light source. Fluorescence testing involves cleaning the stone to be tested and then locating a very dark area for observations. Never look at the UV light source directly, as permanent damage to your eyes can occur. Place the gem on a surface when testing (not between metalllic tweezers or your fingers). Stones that look purplish are inert, with the "purple" color being a reflection of the light source.

To investigate fluorescence further, visit these outside sites:
www.users.interport.net/~kenx/, Ken's fluorescent minerals

Color Filter for Testing Unique Luminescence

A color filter, also called an emerald filter or "Chelsea" filter, can help separate some natural, synthetic, and imitation gem materials. Because color is due to absorption and transmission of different wavelengths of white light, the resulting green, for example, could be a mixture of different wavelengths. These different wavelengths can help distinguish a chromium colored emerald from other green stones and glass imitations, colored by other means. The emerald filter absorbs visible light, except for the long red wavelengths, which are transmitted, causing the emerald to appear red under the filter. Unfortunately, for natural emerald the test is not conclusive, as some synthetics and natural emeralds will not react. Cobalt colored, synthetic spinel, a common aquamarine and topaz imitation, will also appear red with the filter, whereas the the natural gems will appear greenish. An imitation of turquoise, dyed blue howlite, may react red, but natural and synthetic turquoise will appear greenish.

Read more about the Chelsea Filter at JewelInfo4u.com, http://www.jewelinfo4u.com/Chelsea_Filter.aspx and MineralLab.com, http://www.mineralab.com/Chelsea_Filter.htm. Other filters include the synthetic emerald filter (http://www.mineralab.com/Emerald_Filter.htm, tanzanite filter (http://www.mineralab.com/Tanzanite_Filter.htm), and other specialty filters (http://www.mineralab.com/Gem_Filter_Set.htm.

Photo date 1/2000; © by S.W. Aber
For additional information visit
  • Jadeite by Kim Howard, Gemology World, Canadian Institute of Gemology, http://www.cigem.ca/431.html is a fine article on jade. It includes reference to how the color filter can help to identify jade; although it is colored with chromium, it does not appear red through the filter unless it has been dyed or treated, sandwiched in doublet or triplet with colored glue.
  • Cobalt Magic, Blue spinel turns heads with new top-grade material on the market by Diana Jarrett, January 2006, http://www.ganoksin.com/borisat/nenam/blue-spinel.htm. Cobalt Magic shows the value of a color filter and is a ColoredStone article reprinted on Ganoskin.
  • Another use of the color filter was to authenticate a gift, Emerald Man, to the Hudson Museum, The University of Maine, http://www.umaine.edu/hudsonmuseum/emer.php. Also, results of the tests are written up in Archaeology magazine, Volume 51 Number 4, July/August 1998, http://www.archaeology.org/9807/newsbriefs/emerald.html, Emerald Man, by Stephen Whittington, James Vose, and Charles Hess.
  • JAIC online, http://aic.stanford.edu/jaic/articles/jaic28-02-007_indx.html. This is a note regarding the color filter aiding in identification of paint pigments in the art world! An interesting use of a simple instrument when authentication and conservation of paintings is important. Note that this an editors note from 1989 and the color filter is still available from the source mentioned but more expensive than this quote.
  • JAIC online, The pigments of the canosa vases: A technical note by D.A. Scott and M. Schilling, http://aic.stanford.edu/jaic/articles/jaic30-01-004.html, JAIC 1991, Volume 30, Number 1, Article 4 (pp. 35 to 40). This is another article as above, on the use of the color filter in detecting the elemental origin of paint.

Refractometer for Testing Refraction

Photo date 1/2000; © by S.W. Aber
When white light interacts with a gemstone, a portion is reflected off the surface, while another portion, that enters the stone, is slowed down and "bent" or refracted. This optical phenomenon gives rise to an important measurable constant termed the refractive index. The refractive index is a ratio of the speed of light in air to the speed of light in the substance. The refractive index of diamond is 2.42, or light travels 2.42 times faster in air than in diamond. To get up to speed on this subject, go back to the optical properties lecture.

An instrument that measures the refractive index is a refractometer. The refractive index is actually read off a numeric scale, which shows a boundary line between a shaded and brighter portion. This line is a measure of the visible light rays striking the gem at the critical angle or where the light is totally reflected back into the opposite quadrant in the instrument, rather than reflected and refracted out of the stone.

The refractive indices of the refractometer's glass hemicylinder (or the surface the stone is placed on) and the contact liquid (a liquid between the stone and the hemicylinder to provide a contact without any outside interference) determine the upper limit of indices that can be read as 1.81. The most precise readings will be obtained using a monochromatic light source, or sodium light. One reading is possible in singly refractive stones, while two readings are possible when the stone is doubly refractive. The removable polarizing filter, when placed over the eyepiece of a refractometer, can show both refractive indices in doubly refractive stones, by rotating the filter (not the stone) if the orientation of the stone and transmission direction are aligned.

Read more about refractometers at JewelInfo4u.com, http://www.jewelinfo4u.com/Refractometer.aspx.

Photo date 1/2000; © by S.W. Aber
A Jemeter, or reflectivity meter shown in the image to the right, can be used with gemstones having of any refractive index, even over 1.81. A flat facet of the gem is placed over the opening and the test button activated. The display is digital.

A different method used to determine the refractive index is with immersion. Immersing gem material in a liquid with approximately the same refractive index, will make the gem nearly invisible. Immersing the gem in a liquid with a different refractive index can make the gem stand out clearly or in high relief. This method is used with unknown minerals and requires crushing the specimen to view the fragments in different refractive index liquids with the aid of a microscope. Obviously this is not the test for cut and fashioned stones and requires a set of refractive index liquids and a high-powered microscope.

The immersion method is sometimes used without powdering the gem, and simply immersing it in different liquids in an "immersion cell" or small dish. The dish is suspended over a white paper that would allow for a black card to be passed beneath it; when observing from above, if the edge of the card seen through the liquid and stone is a straight line the refractive index of the liquid and stone are the same (Hurlbut and Kammerling, 1991, p. 90). If the stone has a lower index, the card's edge seems to move ahead of the edge seen throught the adjacent liquid or just the opposite if the stone has a higher index.

Some common immersion liquids include: water (1.33), ethyl alcohol (1.36), acetone (1.36), glycerine (1.46), olive oil (1.48), xylene (1.49), clove oil (1.53), ethyl debromide (1.54), bromoform (1.60), methylene iodide (1.74). Relief can sometimes be observed when using the heavy liquids for specific gravity testing. When a stone is dropped into the liquid, to observe if it floats or sinks, it sometimes vanishes! That is the liquid and stone are of a similar refractive index.

Polariscope for Testing Refraction/Dispersion

Dispersion is the separation of white light into its component colors. This phenomenon occurs when light passes from air into a denser medium and the velocity slows according to the wavelength. Red light has the longest wavelength, greatest velocity, and is refracted the least; violet light has the shortest wavelength, least velocity, and is refracted the most (Hurlbut and Kammerling, 1991, p. 83). Dispersion emits rainbow light flashes, which are referred to as a gem's "fire." Dispersion in diamond is moderately high at 0.044, but fluorite displays little or no fire, with a dispersion of 0.007.

Gemstones can be divided into two groups, isotropic or singly refractive and anisotropic or doubly refractive. Isotropic or singly refractive gemstones include minerals in the isometric crystal system and amorphous material, such as glass, opal, garnet, and diamond. These gemstones have one refractive index, that is the light is plane polarized, it moves through the stone with an equal velocity in all directions. Minerals falling into any other crystal system (tetragonal, orthorhombic, hexagonal, monoclinic, and triclinic) are doubly refractive or anisotropic. These gemstones have two or three refractive indices, that is the light is split into different directions with the velocity varying with crystallographic axis directions. When light enters a doubly refractive stone or anisotropic crystal, it is separated into two polarized rays vibrating in mutually perpendicular planes. The rays travel at different velocities and two indices of refraction can be measured on a refractometer and detected with a polariscope. The polariscope is the gem instrument used to separate the two groups, using two polaroid filters, an analyzer and polarizer.

If there is any confusion, revisit the optical lecture. Read more on the polariscope at JewelInfo4u.com, http://www.jewelinfo4u.com/Polariscope.aspx. Also, visit Your Gemologist, Robert James, at http://www.yourgemologist.com/Polariscope/polariscope.html and if you are out and about without your official polariscope, check out how Your Gemologist takes his polarized sunglasses apart and ... http://www.yourgemologist.com/Sunglass%20Polariscope/sunpolariscope.html.

Photo date 1/2000; © by S.W. Aber

Dichroscope for Testing Pleochroism

Light that passes through a doubly refractive gemstone or anisotropic mineral is split in different directions with varying velocity. The light is absorbed differently in different vibration directions, resulting in color variation known as pleochroism. In minerals with only two rays, two pleochroic colors can be detected, called dichroism. In minerals with three principal vibration directions (three refractive indices), three different pleochroic colors can be detected, called trichroism (with only two observed in any one direction). In order to see pleochroism, the gem must be: colored (colorless gems transmit all colors of the spectrum of white light), a single crystal (an aggregate of crystals would scatter the light, obscuring the pleochroism), fairly transparent (numerous inclusions would again scatter the light), and viewed in some direction other than parallel to an optic axis.

Read more about the pleochroism and the dichroscope at JewelInfo4u.com, http://www.jewelinfo4u.com/Pleochroism_in_gemstones.aspx, and http://www.jewelinfo4u.com/Dichroscope.aspx.

Photo date 1/2000;
© by S.W. Aber
Pleochroism can be detected with the polariscope, although the calcite dichroscope is the preferred gem instrument to view pleochroism. The dichroscope is a metal tube with a opening at one end and lens at the other. Optical calcite is mounted inside the tube, producing a double image of the square opening. When the gem is held over a bright light source and viewed through the dichroscope, each image has a different color indicating the vibration directions are at right angles and vary in wavelength. When two differing colors are detected, confirm pleochroism by rotating the instrument 90 degrees and the two colors should switch sides on the split image. Trichroism can be detected by changing the orientation of the stone and one new color will be detected alongside one of the colors seen in the other orientation.

10X Triplet Loupe and Microscope for Testing Magnified Observations

An ESU student looking through
a 10X loupe. Photo date
1/2000; © by S.W. Aber
The function of magnification, whether by a hand held lens or the microscope, is to enlarge an image of the object so details become clear on both the surface and interior. The hand lens or loupe is a single lens system or simple microscope, with a variety of magnifications possible. The triple aplanatic or Hastings triplet is a high quality lens made of two external lenses of flint glass (or lead glass), which is cemented to a double convex crown glass (Hurlbut and Kammerling, 1991, p. 113). [Crown glass is common bottle glass made of silica, soda, and lime; while flint or lead glass, the glass used to imitate gems, is composed of silica, soda, and a lead oxide.] Although the hand lens comes in different magnifications, the 10X (10 power) magnification is good for most gem purposes. A magnification higher than 10X creates difficulty in illuminating the stone, and reduces the field of view and depth of field.

For more information on the loupe visit http://www.jewelinfo4u.com/Jewellers_Loupe.aspx.

For a thorough examination, the binocular stereoscopic microscope is a versatile and very effective instrument to view gemstones. Besides the ocular or eyepiece lenses, there is an objective lens, which creates a two lens system to produce a enlarged sharp image. The microscope magnification is calculated by multiplying the objective and ocular magnification (e.g., a 10X objective and 3X ocular produces a magnification of 30 times). Gem microscopes create a "reinverted" image (regular compound microscopes invert the image) and usually have a zoom feature to vary the magnification continuously. The overhead lighting source is fluorescent, while a high intensity light is transmitted up from the base and through the stone. The light in the base is on a rheostat for intensity control and fitted over with an iris diaphragm, which opens and closes controlling the amount of light that can enter the stone from below.
Photo date 1/2000; © by S.W. Aber

The varied lighting is for dark- and bright-field illumination, oblique reflected, and near-vertical illumination. The light passing through the stone allows examination of the interior of the gem, while the exterior is seen best using the light source above the microscope stage. Dark-field illumination is the standard for examining gems for inclusions and fractures. The gem is illuminated with a cone of light passing through the stone rather than directly reaching the objective lens. The gem appears bright with a dark background. Bright-field illumination, or transmitted light, illuminates the stone directly from below and the iris diaphragm controls the opening or the lessens the glare. Inclusions look dark against the bright background. Inclusions easily observed include, liquid inclusions, curved color banding or striae in flame fusion synthetic blue sapphires, color diffusion around inclusions in heat-treated sapphires, and gas bubbles in glass gem imitations (Hurlbut and Kammerling, 1991, p. 118). Oblique reflected or horizontal light requires the high-intensity fiber optic illuminator to light the stone at an oblique angle or horizontally. This lighting brings out inclusions by reflecting light off their surfaces (thin-film effects), internal fractures, internal cleavage, "fingerprints" and flux healing planes, and rutile "dust" in heat-treated corundum (Hurlbut and Kammerling, 1991, p. 119). Near-vertical overhead lighting is used to detect external features, such as abrasion on facet junctions, scratches, pits, nicks, or polishing marks. The illumination should strike the gem at right angles to maximize the reflection and minimize refraction.

Observing surface abrasion provides information on physical properties, and heat treatment and enhancements, while internal features or inclusions determine the clarity grade and value of the gem. Various types of distinguishing or clarity characteristics aid in identification of unknowns, determining potential hazards for cutting and setting gems, and act as the "fingerprint" for every gem material. These clarity features are found with magnification, and are termed blemishes when they are external and inclusions when internal.

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This page originates from the Earth Science department for the use and benefit of students enrolled at Emporia State University. For more information contact the course instructor, S. W. Aber, e-mail: esu.abersusie@gmail.com Thanks for visiting! Webpage created: 1999; last update: 16 September 2012.

Copyright 1999-2012 Susan Ward Aber. All rights reserved.