a gemstone is allochromatic when it is colorless in its pure state. Subsequent color is derived from an impurity (usually a metallic oxide) that is not an essential part of the mineral's chemical composition. Beryl, quartz and corundum are all examples of allochromatic gemstones.
Describes minerals of variable colour. (See idiochromatic)
pertaining to or having photochemical properties resulting from an impurity or from exposure to radiation. · (of a mineral) having no color in itself but bearing colored impurities. Cf. idiochromatic.