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Alpha Gem Lab
Colored gemstones fall into three categories:
As with a diamond, colored gemstones are graded using the 4 C's, although color is the most important C here because it is the first thing that you notice about the gem.
COLOR: We describe color as the hue, tone and saturation of the stone. Hue is the actual color that you see. Is it red? Does it have a modifying second color so you would call it purplish-red or orange-red? This would be written as "R", pR, oR.
Then we look at the tone of the stone. How light or dark is it on a scale of 0 to 11? 0-1-2 are so light that it looks white and 8-9-10-11 are so dark it just looks black. The ideal tone is in the 4-5-6 range.
Saturation is described using a scale of 1-6. How pure is the color? 1-2-3 include some gray or brown, making for a dull appearance. 5-6 on the saturation scale are bright, vivid colors.
In grading colored gemstones, we also look at the brilliance-window-extinction factor, adding up to 100%. Brilliance is the bright sparkle that meets your eye. This should be the largest percentage of the three. The window is usually in the center of the stone. Does the light enter and go straight through (like a window) allowing you to see a pen or finger behind it? Extinction refers to dead dark areas.
SHAPE: This is the outline of the gemstone. It can be emerald-shape, pear, oval, round, square, heart, marquise, fantasy, oval cabochon, or round cabochon. Grading the shape takes into account things like odd measurements that require special mountings, sides not parallel, sides not evenly matched or misshapen, corners that are too narrow or too wide, undefined points, and flat or bulged sides.
CLARITY: Gemstones are classified according to type:
Type 1 (often inclusion-free): Type 2 (usually included): Type 3 (almost always included):
Beryl -Aquamarine, Morganite, yellow Andalusite Beryl - Emerald
Chrysoberyl - green, yellow Chrysoberyl - Alexandrite Tourmaline - red, pink and watermelon
Quartz- smoky Corundum (sapphire and ruby), all colors
Spodumene - Kunzite Diopside
Topaz Garnets - all colors
Tourmaline - green Iolite
Zircon - blue Peridot
Zoisite - Tanzanite Quartz - citrine, amethyst, amethyst-citrine
Tourmaline - blue, green, orange, yellow, bi-colored
Zircon - all but blue
CARAT WEIGHT: Actual scale-weight or determination by recognized formula for the type and shape of the specific gemstone.
Gemstones can be enhanced to stabilize or change their original color. They can be bleached, oiled, heated, irradiated, dyed or impregnated with a substance to make them more durable. Some treatments are widely recognized as "normal" treatments while others are performed to fool the potential buyer. These can have an effect on the value of the stone. Not all treatments are considered permanent and may fade or dry out with time. Not all treatments can be detected with a glance or even with use of a microscope. All should be disclosed by the seller. When in doubt, the gemstone should be sent for testing to an international laboratory such as GIA.
Another service that the international labs may provide is "country-of-origin." In some cases this will also impact the value of a stone.
COMMONLY RECOGNIZED BIRTHSTONES:
January - Garnet (all colors) July - Ruby
February - Amethyst August - Peridot
March - Aquamarine September - Sapphire (all colors)
April - Diamond (all colors) October - Opal
May - Emerald November - Topaz
June - Alexandrite or Pearl December - Zircon (Tanzanite on some lists)