Some are idiochromatic, or "self-colored," meaning that their colors are inherent in the chemical and physical makeup of the materials themselves. For example, azurite is always blue and malachite is always some shade of green. Other minerals are allochromatic, or "other colored," meaning that their colors are acquired through contamination by chemical impurities. For example, pure beryl is colorless.

The pres¬ence of a trace of chromium turns it green (emerald); iron impurities color beryl blue-green or blue (aquamarine). Combinations of such impurities create a wide range of colors in various gemstones.For the idiochromatic chromatic minerals, consistency makes color a useful identifying feature. But for other minerals that can be colored by various chemical impurities, color can be a very misleading and unreliable characteristic. For the gem buyer this problem is compounded by the limited range of colors exhibited by the commercially popular stones found in most jewelry shops.

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To most people garnet is a red gem, yet it occurs in shades of purple, orange, pink, yellow, and green. Jade is not only green, but can be blue, purple, red, brown, gray, white, or yellow. In gems, subtleties of color can mean enormous differences in value.For most gemstones, depth of color and richness of color are synonymous with higher value. This is especially apparent in the case of topaz, aquamarine, and amethyst. But too dark a gem color can actually detract from its value, as in the case of green tourmaline, blue sapphire, and even some emeralds.