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Color in Gemstones
Color is the most conspicuous quality of a precious gemstone. The levels of selective light absorption determine the color of a gemstone. Colorless gems transmit all wavelengths of light, while black stones absorb all the light. The frequency of its wavelength is what gives light its apparent color. The partial absorption and reflection of certain wavelengths of the electromagnetic spectrum produce stones featuring all the hues of the rainbow. The colors of gemstones are important factors in their identification and evaluation.
Allochromatic Gemstones
Most colored gemstones are called allochromatic by gemologists. Impurities in the stone give them their color. The impurities, known as transition elements or chromophores, leach into the gemstone from the surrounding matrix as it forms. Allochromatic stones vary a lot in their color tones and shades depending on the amount of impurities they contain. A pure allochromatic stone is often colorless. Examples of stones that can be almost any color are beryl, corundum, quartz, tourmaline, topaz, garnet and jade.
Idiochromatic Gemstones
The other main type of colored gemstones is called idiochromatic and get its color from transition elements that are essential components of the stone's chemical makeup. Since these gemstones share a common element in their chemical formula, all idiochromatic stones occur in a narrow range of shades of the same basic color. Elements that impart color to idiochromatic stones include the chromophores: titanium, iron, copper, nickel, chromium and manganese. Self-colored gemstones include cuprite, malachite, peridot, azurite and rhodonite.
Pseudochromatic Coloration
Diamonds and zircon get their apparent coloration not from chemistry, but from optical effects generated by the mineral's physical structure. Called pseudochromatic coloration, the appearance of color is from the refraction and spectral dispersion of light. Defraction of light creates the opalescent colors of agate and opal. Labradorite gets its color from an effect known as iridescence. Pearlescent effects give pearls, talc and gypsum their luster. An effect known as pleochroism causes gems such as alexandrite and rubellite to appear to change color when viewed under different kinds of light.