Idiochromatic Gems

A mineral whose own particular composition and structure are responsible for the colors that are passed through or reflected back is said to be "idiochromatic." The gemstone peridot is idiochromatic because it is always green and must be green because of what it is, chemically and physically. On the other hand, beryl can be yellow, green, blue-green, colorless, or even pink. Absolutely pure beryl is colorless. The colors in this case arise from the presence of trace amounts of impurities.

Minerals with variable color depending on trace impurities, or sometimes on structural imperfections, are said to be "allochromatic." Both idiochromatic and allochromatic colors are caused by the interaction between light and some of the electrons that are part of the atoms that compose the mineral or are present as impurities. Atoms whose electrons commonly cause color in solids are usually of a group of elements called transition elements. These include copper, iron, manganese, chromium, nickel, cobalt, vanadium, and titanium. When these coloring agents, or "chromophores," are present in a gem the color is quite stable, but it can be changed under any conditions, such as excessive heat or radioactivity, which will change the nature of the chromophore.