OPTICAL PROPERTIES
COLOR IS THE MOST OBVIOUS visual feature of a gem, but in fact it is just the optical properties of each gem one of many optical properties, all of which are dependent upon light. The individual crystalline structure of a gemstone interacts with light in a unique way, and determines species. Effects produced by light passing through a gem are described here; those produced by the reflection of light.
What makes color?
The color of a gem depends largely on the way it absorbs light. White light is made up of the colors of the rainbow (spectral colors), and when it strikes a gem some spectral colors are "preferentially absorb- ed". Those that are not absorbed, pass through or are reflected back, giving the gem its color. Each gem in fact has a unique color "fingerprint" (known as its absorption spectrum), but this is only visible when viewed with a spectroscope . To the naked eye, many gems look the same color.
ALLOCHROMATIC GEMS
Allochromatic ("other-colored") gems are colored by trace elements or other impurities that are not an essential part of their chemical composition. Corundum, for example, is colorless when pure, but impurities in it (usually a metal oxide) create the red stones we know as rubies, blue, green, and yellow sapphires, and orange-pink padparadscha. Allochromatic gems are often susceptible to color enhancement or change.
DIOCHROMATIC GEMS
The color of idiochromatic ("self- colored") gems comes from elements that are an essential part of their chemical composition. Thus idiochromatic gems generally have only one color, or show only a narrow range of colors. Peridot, for example, is always green, because the color is derived from one of its essential constituents, iron.
PARTI COLOURED GEMS
A crystal that consists of different-colored parts is called parti-colored. It may be made up of two colors (bicoloured), three (tricoloured), or more. The color may be distributed unevenly within the crystal, or in zones associated with growth. The many different varieties of tourmaline probably show the best examples of parti-coloring, exhibiting as many as 15 different colors or shades within a single crystal.
PLEOCHROIC GEMS
Gems that appear one color from one direction, but exhibit one or more other shades or colors when viewed from different directions, are known as pleochroic. Amor- phous or cubic stones show one color only; tetragonal, hexagonal, or trigonal stones show two colors (dichroic); orthorhombic, mono- clinic, or triclinic stones may show three colors (trichroic).
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