Diamond, ruby, sapphire, emerald, and topaz are all central to a jeweler's inventory. Diamond has already been presented and this lecture will focus how colored gemstones are evaluated. After learning about evaluation grading, you will want to investigate individual gemstones in more detail.
|Very few diamonds over one carat are ever sold without a
diamond grading report, which certifies the stone's authenticity
and evaluates the color, cut, clarity, and carat size (see a
sample report, provided by R. Genis). These reports are issued
by organizations such as the Gemological Institute of
America and the Gem Trade Laboratory, the American Gemological
Laboratory, and the European Gemological Laboratory. To see the newest generation of diamond grading, colored diamond grading, and gem identification report forms visit http://www.gia.edu/gemtradelab/110/reports__services.cfm. Colored stones are also graded, but in a less
systematic way (see a sample report,
provided by R. Genis). Unfortunately, there is a paradox with
evaluating colored stones in that often times the most qualified
people, gem dealers, are also the most biased regarding their
Although no universal standard for colored stone grading exists, there is an increasing need for reports to verify identity and genuineness due to the prevalence of synthetics, composites, enhancements, as well as the variety of colored stones on the market today (Matlins & Bonanno, 1998, p. 119). There is dissent though, among reporting labs, on whether or not to disclose a gem's country of origin and lab-induced color enhancements. A gem's geographic origin may not be positively known; for example, Thailand is still the major country for the marketing of ruby, but little, if any, ruby is mined in Thailand today (Hughes). Regarding enhancements, some argue that colored gemstones have been routinely treated for centuries and therefore noting such treatment is unimportant (investigate gem enhancement more, from the American Gem Trade Association).
Image taken from
a former site at
sponsored by The
Diamond Trading Company
Image taken from The Image,
showing sapphire colors.
|Diamond and colored stone evaluation and pricing is based upon
quality, beauty, and rarity. Colored stone quality is determined in
a similar manner as diamonds, by the 4 c's, color, clarity, cut,
and carat. Cut is the most important factor in diamond evaluation,
whereas color is key in other gemstones. The more beautiful and
rare the color, the less important clarity and carat are to the
overall value of the gem.
In describing color, the hue, intensity, tone, and distribution are all considered. Hue is simply the color, whereas intensity is the saturation or degree the color varies from achromaticity. Tone is the degree of lightness or darkness, and distribution refers to color zoning within the gem. Intensity and tone may be affected by cut, and the darker depth of color is usually more desirable, although it will negatively affect dispersion (fire). See the Gem Hut for excellent coverage of colored gemstone color, http://www.gemhut.com/info.htm. The overall color may be variable within a gem species or variety because of inclusions, degree of transparency, and strength of fluorescence and pleochroism. When judging color, a plain white background is best, as the background color may mask a less desirable hue. For example, rubies are often shown on brass plates, yellow table tops, or in yellow cellophane-lined papers, to mask the bluish tint, making the gem appear more red (Hughes). Conversely, colorless diamond is often shown with blue cellophane-lined paper to mask the yellow tint!
Clarity refers to the presence or absence of internal or external flaws and blemishes. Flawlessness is not as desired in colored gems as it is with diamond. Imperfections may aid in identification and origin of the gem, as well as proving the gem to be natural or synthetic. The type and placement of flaws is important though, and numerous flaws visible with the unaided eye in a face-up position may decrease the value. Flaws that might affect durability will decrease the value also. See the Gem Hut for excellent coverage of colored gemstone clarity, http://www.gemhut.com/info.htm.
Any cut, which refers to both shape and proportions, should maximize a gem's beauty. Colored stones are either faceted or cut en cabochon. A cabochon cut is an unfaceted style often used with translucent to opaque gems. The faceted step cut is often used with colored stones, and may be the best cut to highlight the hue and depth of color, although not maximize scintillation and brilliance. Faceting proportions and simply the number of facets can affect brilliance (liveliness) and scintillation (sparkle). Larger gems will have greater scintillation (sparkle), when there are more facets for light to interact with. In general, a shallow crown reduces scintillation and a shallow pavilion creates a see-through window. A pavilion that is too deep (often done based on the shape of the rough and/or an effort to increase carat size) will create darkness and dead areas. Some general faceting mistakes include asymmetrical girdle outline, a wavy girdle, table not parallel to the plane of the girdle, off-center table or culet/keel line, facets not meeting at a point or misshappened, and rounded facet junctions.
Carat is the weight measure for gemstones, with the exception of pearls and coral. One carat is equal to 200 milligrams or 0.2 grams. Pearls and coral are sold by the grain, momme, and millimeter. A grain is one-fourth of a carat, 0.25 carat or 0.05 grams, and a momme is 18.75 carats. Are all one carat gemstones the same size? No! Carat, grain, and other such weight designations should not be confused with the gem's size, which is determined by the specific gravity. For example, diamond has a higher specific gravity than emerald and therefore a one carat emerald will be larger than a one carat diamond, when placed side-by-side, and both are in the same shape and proportions.
Having a brief background in colored stone evaluation will help to understand and appreciate why gem quality corundum, beryl, and topaz are sought after. Each have desirable colors, are relatively rare, and even though they are often flawed, inclusions are only a negative factor in diamond. Diamond, ruby, sapphire, emerald, and topaz, are 10, 9, 9, 8, and 8 on the hardness scale and each have relatively high refractive indices, creating small critical angles. With a proper cut, the high hardness and refractive indices help to create long lasting beauty and quality.
Return to the Syllabus or go on to one of the gem sites shown below.
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: email@example.com Thanks for visiting! Webpage created: December 7, 2003; last update: March 9, 2008.Copyright 1999-2008 Susan Ward Aber. All rights reserved.