Colored Stone Grading - Phenomenal Gems
Light behaves in unique and interesting ways in certain gemstones. While it dances and sparkles as it leaps out of a diamond or simmers rich red as it passes through a fine ruby, the light we see reflected from a fine opal, moonstone, cat's eye, or star sapphire has been transformed by the gem material into magical displays that we call optical phenomena.
Each different optical phenomenon has a specific name and cause, and each presents its own grading challenges. In gemology, we call these wonderful stones the phenomenal gems and we grade them by evaluating the overall quality and sensory impact of the optical phenomenon present. To display the optics effectively, most of these gems are cut as cabochons, so the cut grade is for shape, polish, and appeal. Brilliance is not applicable to the cut grade of a cabochon.
Play of Color
In some types of opal, light is dispersed and diffracted by structures within the opal into patches and streaks of rainbow colors that move and change with the light. This phenomenon is known as play of color. Opal that shows play of color is probably the most popular and commerically important of the phenomenal gems and is sometimes referred to as precious opal. Because opal shows an extremely wide variety of types, colors, intensities, and patterns, it requires a more systematic grading approach than other phenomenal gems.
Adularescence
When a gem is displayed under a source of light, this phenomenon (also known as schiller) is a billowy glowing light that emanates from within the gemstone. In moonstones, light interacts with thin transparent layers of alternating orthoclase and albite feldspar within the gem. Slight differences in the refractive indexes of the feldspars scatter the light - the thinner the layers, the bluer the sheen.
Moonstones, rose quartz, prehnite, and agate are gems that exhibit adularescence. Moonstone is the most well known and popular. While it is usually cut as a cabochon, we also see fine carved examples. Full grading applies mainly to moonstone because of its value.
Grading Adularescence
Grading adularescence begins by evaluating the strength of the effect. We assign a number grade as follows:
- Faint
- Weak
- Moderate
- Strong, Bright
- Very Strong, Very Bright
Next, we grade the color of the phenomenon by description. Blue and Rainbow are generally ranked as the best sheen colors in moonstone, but rare specimens may exhibit unusual tones including orange. White is the most common color and least valuable.
The orientation of the effect is very important- it should be centered in the middle of the stone at its highest point, and correspond to the shape of the cabochon. The cutter must orient the material properly before he cuts the cabochon shape for the effect to display correctly.
Lastly, we describe the body color and transparency of the gem. Colorless moonstones that are also highly transparent show their glow to best effect.
Iridescence and Labradorescence
These phenomena refer to the "rainbow" effects, somewhat similar to oil on water. This is caused by interference of light by structures within the gem materials. Labradorite, a variety of orthoclase feldspar, fire agate (pictured above), mother of pearl, and iris agate exhibit this phenomenon. The orient of pearls is a related effect. These effects are rarely graded specifically.
Chatoyance
Also known as cat's eye, chatoyance is similar to adularescence. When viewed under a source of light, the chatoyant gem manifests a bright and distinct line, preferably centered on the stone. Chatoyance is usually caused by minute fibrous inclusions within the stone that are parallel to each other. The lens-like shape of the cabochon focuses light onto the fibers where it becomes collected and reflected back as the bright line that we call the "eye." Occasionally, factors other than inclusions will cause a cat's eye.
When the stone is rotated in light aimed from about 45 degrees, the eye will actually open into two distinctly bright sides with a darker center. This dramatic effect is much like what we sometimes see in the eye of a cat, hence the name. It will open and close as we rotate the stone back and forth through 90 degrees.
If the light is aimed at the gem parallel to the bottom, many chatoyant gems will show a "milk and honey" effect, where the side opposite the light source is distinctly brighter than the closer side. This is caused by light being carried along the parallel inclusions to the opposite side.
The classic and generally most valuable cat's eye gem is chrysoberyl, and the most common and least expensive is tiger eye. We also see many other chatoyant varieties of gems including alexandrite, aquamarine, emerald, tourmaline, garnet, diopside, and others.
Grading Chatoyance
We grade the sharpness of the eye, its orientation, whether it's complete and straight, and the milk and honey effect if present. Generally, the finer the texture of the inclusions that make the eye, the sharper it will be.
Asterism
Known as the star effect, this phenomenon is basically crossed chatoyant bands of light displayed on the highest part of the cabochon. Caused by tiny needle-like inclusions that are oriented in several specific directions, the number of "legs" on the star varies among the different gem species.
Star rubies and sapphires, the most valuable star stones, normally display 6 legs. Star garnets and diopsides/enstatites typically have 4 legs. Occasionally we see gems with double the normal number of legs, an effect caused by twinning within the crystal structure of the gem. Both cat's eye and star gems will show multiple eyes or stars under multiple light sources.
Grading Asterism
With star stones, it's very important for the star to be centered on the high point of the cabochon and for the legs to be straight, evenly spaced, and complete. We apply similar criteria as with cat's eyes - the brightness, sharpness, orientation, number and straightness of legs are evaluated.
Color Change
A number of gems actually change body color when viewed in different types of light. For example, a fine alexandrite (shown above) may look purplish red when seen in incandescent light, but a slightly bluish green when seen in daylight. This phenomenon is caused by selective absorption - light passing through the gem is "filtered" by a "trap door" in the middle of the spectrum. When viewed in light with a warmer color temperature, the gem will be seen on the red side; and when seen in light with a cooler color temperature, it will appear more green.
The classic color change stone is alexandrite, but sapphire, garnet, and a few other less common gems including diaspore will show color change. The best alexandrite can show perhaps the most dramatic shift, from red to green.
Grading Color Change
Color change gems are graded in incandescent light and then in daylight. The degree or strength of the color change is graded as strong, moderate, or weak.
If the color change is weak, it will have little effect on the value. If it is strong, it can greatly increase the value in an otherwise fine and rare gem.
The purity and intensity of the colors have a very significant effect on the desirability of a color change gem. For example, Sri Lanka produces some very large alexandrites with a moderate to strong color change, but also with a noticeable brownish overtone resulting in a change that is less dramatic. These brownish-looking stones are much less desirable than smaller gems with strong change and pure bright colors.
The separation of the colors on the spectrum is very important. A good color change sapphire goes from violetish blue to reddish purple, while alexandrite dramatically leaps from red to green. The alexandrite is much more sought after for its change.
Faceted color change gems are graded for body color, clarity, and cut just as are other faceted gems.
Grading Clarity
Since phenomenal gems are prized for their display of a particular phenomenon and not for light return as we see in faceted stones, clarity grading is generally not applicable to these gems. In fact, the presence of inclusions is what creates the phenomenon. Excessive inclusions may lower the overall quality grade. For example, excessive silk in certain types of star sapphire, star ruby, and tiger eye gives the material an almost opaque appearance, lowering the overall appeal. The presence of inclusions not associated with the light phenomenon can also detract from the appearance and appeal of a gem, and these will be accounted for in the overall quality grade.
Grading Body Color and Transparency
We grade the overall body color and transparency of the gem. Adularescent, chatoyant and star gems are most beautiful when they are rather transparent and limpid, with just enough inclusions to generate a strong effect. The "texture" of the body of these phenomenal gems is very important in overall quality, especially in the top quality gems. The color of star rubies and sapphires is considered just as it is in faceted material.
Rarely, we see gems that combine optical phenomena, such as Alexandrite Cat's Eye or Color Change Star Sapphire. The merits of each phenomenon add to the value of the gem, sometimes substantially.
Cut Grade
Finally, we grade the cut of the gem. Since most phenomenal gems are cabochons, we look at the shape and polish and the overall beauty and effectiveness of the cut of the cabochon. We assign individual grades for both cut and polish, and an overall cut grade:
- Excellent
- Very Good
- Good
- Fair
- Poor
Overall Quality Grade
On a scale of 1 to 10, we rate the combined effects of phenomena, body color, transparency, and cut, as well as rarity. Since the effect of the phenomenon is the dominant feature for grading, we start there and then apply adjustments for the other factors.
Cut is less important in grading these stones than for transparent faceted gemstones. So, a stone graded 9 for phenomena could have just good cut grades and still be considered Extra Fine 9. Conversely, a stone graded 8 for phenomena, with excellent cut and polish might be graded overall as 8.5.
In our appraisal and grading practice, we follow the GIA Gemstone Color Description System and the general grading/pricing structure found in the Gem Guide wholesale gem pricing book published biannually by Richard Drucker, Gemworld International Inc., because we find it to be the most reliable and consistent source of market based price data available for colored gemstones. Since price follows grade, or vice versa, our quality grades will reflect our opinion of the position of a gem within the market place.
- 8 - 10: Extra Fine. The best of the best, rarely seen, and never cheap. Phenomenon is always very bright and strong in all directions, sharp, perfectly oriented and complete. Body color should be top quality for type. May have minor to slight defects in cut, but generally, the cut is superb, reflecting the quality of the material. These stones are often truly unique.
- 6 - 8: Fine. Phenomenon may be bright or very bright, may have minor to slight defects in effect, less than ideal inclusions. Good to Very good body color for type. May have slight defects in cut, should not be too thin or bulky on the bottom.
- 4 - 6: Good. Phenomenon is moderate to bright, some parts of the effect lacking, fair to good orientation, may have many inclusions. Fair to Good body color for type. May have moderate defects in cut.
- 1 - 4: Gemstones that fall into this category are what the trade calls "commercial". Can include stones with weak or incomplete effect, some dull areas face up. Low commercial may be very thick or otherwise poorly cut, weak or dull phenomenon, and/or deficient in color.