Gemstone Guide

Gemstones are evaluated using the same 4Cs we discussed in our diamond guide: color, cut, clarity and carat weight (size).

Gemstones are compared to other stones of the same type. The rarity of a gemstone affects the value. Treatments can either increase or decrease the value compared to a gem that is natural and untreated.

Inclusions can also either increase or decrease gemstone value. For example, a star sapphire is more valuable because of its inclusions. Inclusions can also indicate locality, where that gem was mined.

Values are affected by provenance, meaning where the stone came from and its history. Finally, demand for a particular gemstone increases the price.

Gemstones by color

If you are not sure what kind of gemstone you have, start with the color.

Color	Gemstones
Black	Onyx, Black Opal (very rare), naturally black Tahitian Pearls or treated Akoya or Freshwater Pearls
Blue	Aquamarine, Alexandrite, Blue Chalcedony, Blue Sapphire, Blue Spinel, Blue Tanzanite, untreated Blue Topaz (extremely rare), Blue Tourmaline, Blue Zircon, Lapis, Natural Treated Topaz
Brown	Brown Citrine, Brown Diamond, Fire Agate, Chocolate Opal, Tiger’s Eye, Mahogany Obsidian, Smoky Quartz, Brown Topaz, Brown Tourmaline, Golden Brown Zircon
Gray	Gray Agate, Gray Chalcedony, Gray Diamonds, Gray Fluorite, Gray Hematite, Gray Labradorite, Gray Moonstone, Gray Opal, Gray Sapphire, Gray Spinel, Gray Tourmaline, Mother-of-Pearl, Tahitian Pearls
Green	Bloodstone, Emerald, Green Garnet, Jade, Jadeite (very valuable untreated), Peridot, Green Sapphire, Green Tourmaline, Green Zircon
Pink	Pink Garnet, Morganite, Pink Sapphire, Pink Spinel, Pink Tourmaline, Rhodolite Garnet
Purple	Alexandrite, Purple Amethyst, Purple Sapphire, Purple Tanzanite, Purple Zircon
Orange	Orange Garnet, Fire Opal, Orange Sapphire
Red	Red Garnet, Fire Opal, Red Spinel, Red Tanzanite, Red Tourmaline, Ruby
White	Akoya Pearl, Freshwater Pearl, Mabe Pearl, Moissanite, White Opal, South Sea Pearl, White Spinel, White Topaz, White Zircon
Yellow	Citrine, Fire Opal, Yellow Sapphire, Yellow Zircon
Multi-colored or translucent	Alexandrite, Iolite, Boulder Opal, Ethiopian Opal, Jelly Opal, Sunstone

If you aren’t sure what kind of gemstone you have, we can determine that for you.

Gemstone shapes

Gemstones come in all shapes and sizes. The most common are:

• Round
• Oval
• Pear or teardrop
• Heart
• Princess
• Emerald
• Marquise
• Radiant
• Asscher
• Royal Asscher
• Baguette
• Cushion
• Octagon
• Triangle
• Square
• Trillion
• Star
• Flower

Gemstones are cut in the most flattering shape for each type of gem. The shapes above can be faceted to create complex shapes such as checkerboards, or cut as cabochons.

Gemstone enhancement

As mentioned previously, enhancements can increase or decrease the value of a gemstone. The FTC requires sellers to clearly disclose key specifics about gemstones:

“With the exception of the normal fashioning (cutting and polishing) of a gemstone, it is the seller’s responsibility at all levels of commerce to clearly disclose to the buyer at the time of sale:

1. Whether the gemstone is natural or not;
2. All information pertinent to any enhancement process done to a natural gemstone when: (a) the treatment is not permanent and its effects are lost over time; or (b) the treatment creates special care requirements for the gemstone to retain the benefit of the treatment; or (c) the treatment has a significant effect on the value of the gemstone.”

The American Gem Trade Association (AGTA) advises that if a treatment is not known, but suspected because such stones are commonly treated, the FTC states that “it is prudent and appropriate to disclose gemstone treatments” that may apply.

Gemstone enhancement codes

Many gemstones are treated to bring out attractive qualities or make them more durable. The following are the gemstone enhancement codes:

Code	Enhancement	What it means
N	Not enhanced	Natural stones which are not commonly enhanced. If a gemstone is commonly enhanced, the seller must provide a guarantee it is natural to use the “N” symbol.
E	Enhanced	Traditional enhancements for that gemstone.
ASBL	Assembled	Assembled from multiple stones.
B	Bleaching	Chemically treated to bleach or remove color.
C	Coating	Coatings or films to improve appearance.
D	Dyeing	Colored dye to improve or change the color.
F	Filling	Flux or similar clear filling to improve appearance, increase durability or add weight.
H	Heating	Heat used to increase phenomena or alter color or clarity.
HP	Heat and pressure	Heat and pressure used to increase phenomena or alter color or clarity.
I	Impregnation	Surface impregnated with a filling to add weight or improve appearance or durability.
L	Lasering	A laser or chemicals used to enhance the clarity of inclusions inside, typically on diamonds.
O	Oiling/resin	Improving the appearance by filling surface fissures with a colorless substance such as resin, oil or wax, but not plastic or glass.
R	Irradiation	Irradiation to enhance the color or reduce the visibility of inclusions.
U	Diffusion	The use of high heat and chemicals to induce a thin layer of color or asterism (as in star rubies).
W	Waxing/oiling	Improves the appearance of opaque stones by infusing a colorless oil, wax or paraffin.
SC	Special care	Prevents loss of enhancements, usually by avoiding light, chemicals, heat and ultrasonic cleaners.
CMP	Composite	Composite stones are created using more than just a gemstone, often lead glass or polymers.

These codes and treatment types must be used as directed in the Gemstone Information Manual. The American Gem Trade Association provides codes which are fully explained starting on page six of the Gemstone Information Chart.

Selling a gemstone?

Our specialists identify gemstones, check for treatments and value them at no cost. See what we buy, read about metals used in jewelry, or ask for a free evaluation.