Gemology 101: A Crash Course for the Counter

It was the winter of ’76 (1976, that is), and my jewelry-store-owner father had just overheard my feeble attempt to sell a quarter-carat diamond ring to a young couple. I was out of college and working in the store full time, but I was obviously not ready for prime time at the diamond counter or any other gemstone counter. After watching me blow the diamond sale, Dad pulled me aside and said: “You’ve got a choice of New York City or Santa Monica, California.” I just stared at him, uncomprehending. “You’re going back to school,” he said. “This time to GIA (the Gemological Institute of America).”

An education in gemology is one of the finest gifts you can give your sales staff and one of the best investments you can make in your business. But when you’re trying to get geared up for holiday sales-especially if some of your staff are new-you may not have the luxury of time to enroll them in gemology courses.

So here, just in time for the holiday selling season, is a brief checklist of some gemology basics. It is by no means a comprehensive education, but it will help your staff avoid some of the worst gemological gaffes. Use it to help prepare new staff for holiday gemstone sales, and keep it handy for a quick and easy reference for seasoned staff.

Don’t assume that an old stone has to be natural. Synthetic rubies were made at the turn of the 20th century, andsynthetic sapphires have been around almost as long.


Diamond is the hardest gemstone on the planet, but a diamond can break! “Hard” means nothing else can scratch it. Diamond is also quite tough, which means it can resist most blows, but it’s not the toughest gemstone in the world. One quick strike in the wrong spot against a car door can chip or break a diamond. (Jadeite is the toughest gem you have in your case. It resists breaking so well that it can be carved into a ring or bangle bracelet and worn without worry.)

The “four Cs”

This term refers to the methods by which diamonds are evaluated. They are color, cut, clarity, and carat weight. Clarity refers to the presence (or lack!) of flaws, called inclusions, in a stone. Some diamonds are flawless, but the chances that you have one in stock are slim. There are no “bubbles” or “carbon spots” inside diamond, but inclusions may be crystals of diamond (or other minerals) that look like bubbles, or dark minerals that look like charcoal. Try to familiarize your staff with GIA’s clarity grading scale, which assigns a grade range based on how many and how visible the inclusions are in each diamond. Cut refers to the angles of a diamond’s facets. (Example: crown angles of 34.5°.) It does not-as some consumers may believe-refer to the shape or style of cutting, such as round or emerald cut. Facet angles affect the way light is returned from a diamond, which determines whether the stone appears bright or dark and whether it flashes with spectral colors. If the cutter did a fine job, the diamond will possess an aesthetically pleasing balance between brightness and flash of colors. Carat weight is not the same as size, although most people equate the two. In fact, one 2-ct. diamond might look bigger than another 2-ct. diamond because of the way each was fashioned. A well-cut 1-ct. diamond would measure 6.5 mm in diameter, whereas one that isn’t cut to proper proportions might measure 5.8 mm or 7 mm-but both 1-ct. diamonds will weigh the same-one carat, which equals .2 grams. (You can put nearly 144 carats of diamonds in an envelope, stick a 33-cent stamp in the corner, and mail it First Class-but I don’t recommend it.)

Diamonds come in all the colors of the rainbow, ranging from faintly tinted to brightly saturated. Color, as most consumers understand it, actually refers to relatively colorless diamond. Most diamonds aren’t really white-the so-called “white” stones you have in stock would be graded colorless or near colorless (D through J) on the GIA scale. In GIA color grading nomenclature, categories begin at D (colorless) and continue through the alphabet as color increases, all the way to Z. (When GIA devised its system, many dealers were using A, B, and C to classify their stock, so GIA started at D to avoid confusion.)

If a diamond has more color than one graded Z, it’s classified as a fancy colored diamond, which has its own evaluation criteria.


The more common fancy diamond colors are yellows, browns, and grays; rarer are pinks, purples, oranges, blues, greens, and reds. There are some very rare whites (like milk), and even black diamonds.

Some diamonds get their color naturally, while others are enhanced by irradiation. (The finished stone is not radioactive.) A .90-ct. natural colored green diamond recently sold for more than half a million dollars. An irradiated green diamond costs considerably less, but the stone is still a natural diamond-it’s the color that isn’t natural.


Gems are either natural (made by Mother Nature) or synthetic (created in a laboratory). There are synthetic diamonds, rubies, emeralds, and sapphires, and there are synthetic varieties of almost every other gemstone you have in your showcases. Chemically speaking, synthetics are exactly the same compounds as their natural versions. The only difference is in where the gem originated-in the earth or in a laboratory-which affects the price. An inexperienced sales associate may think that because a gem has an important-sounding name, it must be natural. Burma ruby, Colombian emerald, and Ceylon sapphire are descriptions of natural gems. A Chatham emerald is a synthetic. A Linde star sapphire is a synthetic. A Gilson opal is a synthetic. Trade surveys estimate that half of your amethyst inventory may be synthetic. Consider marking tags so your staff won’t have to guess which gems are natural and which are synthetic.


Many gemstones are enhanced to improve their apparent clarity or color. Rubies and sapphires are routinely heated. The vast majority of emeralds are filled with oil or resin. Diamonds can be filled, lasered, and subjected to high pressure and high temperature. Topaz and tourmaline are commonly irradiated. Mark all tags so your staff will know which gems have been treated (or “enhanced,” to use the euphemism). If you have gems whose color is natural, point them out to your staff-those gems are rare, and that rarity is an important selling tool.


Virtually all tanzanite-99.99%-starts out brown and only acquires its blue/purple color through heat treatment. An ultra-rare blue/purple tanzanite of natural color most likely will come with a written guarantee or a laboratory certificate stating so.


Cultured pearls, including Japanese akoya, Tahitian, and South Seas, are nucleated (i.e., implanted) with a mother-of-pearl bead. Thus, except for the layer of new pearl material called nacre, much of the pearl is bead. Nacre thickness varies. Tahitian and South Seas cultured pearls will have thicker nacre than akoyas. Chinese freshwater cultured pearls are nucleated with a small piece of tissue taken from another mussel, which results in gems that are nearly all nacre. Natural pearls, which are rare, are composed of all nacre.

Pearl Enhancements

Don’t forget to include your pearl showcase in enhancement discussions. Almost all “white” akoya cultured pearls are bleached using a compound similar to hydrogen peroxide. Some colored cultured pearls are dyed.

Gem care

Each gem requires different care. Make sure your staff knows which gems are durable and which are not, and make sure they understand-and can explain-what precautions buyers should take with various gemstones. Opals, for example, are delicate and easily scratched, cracked, or broken. Soaking them in water won’t prevent cracking and may even hasten damage. Kunzite (pink spodumene), and morganite (pink beryl) may fade if left in the sun (e.g., kept in bright showcases, or worn at the beach too long). Enhancements may be affected by improper care. Oil in emeralds may leak out if the gem is kept for weeks in a hot showcase, unless a hardener also has been added. The American Gem Trade Association’s Gemstone Information Manual has information on which gems are enhanced and explains what precautions are necessary to keep gems looking new.

“Grandmother’s Ruby”

If a customer comes in with an old stone, don’t assume that because of its age, it has to be natural. It might be a synthetic. Synthetic rubies were made at the turn of the 20th century, and synthetic sapphires have been around almost as long. The “alexandrite” that was purchased during a military tour of duty or during a Mexican vacation is more than likely a synthetic sapphire that changes color. Make sure your staff never identifies somebody’s gemstone unless they’re trained to do it. Always ask the gemologist on staff to take a look.


Know them all, including the alternatives. January’s garnet, for example, is common red almandite, but garnets of other colors-such as purple “grape” garnets, Kenyan green tsavorites, and Nigerian yellow-orange spessartites-may be perfect substitutes to suggest to a customer who isn’t crazy about the red color typically associated with garnet.

Diamond “simulants”

Cubic zirconia and synthetic moissanite are not synthetic diamonds. They may resemble diamonds, but they’re completely different minerals. There is synthetic diamond that looks exactly like diamond-because it is diamond. But it’s made in a laboratory, not by Mother Nature.

Malaia garnets

It’s pronounced Malaya, but they’re not from Malaysia, they’re from Africa.

When in doubt, ask. Make sure your staff feels comfortable asking questions about anything they don’t understand. It’s better to have them ask the same question a dozen times than to give a customer incorrect information. Keep reference books handy for them to check, and if a question stumps you, contact JCK’s gemstone editor, and we’ll try to help. Call (610) 205-1107 or e-mail