But there are many minerals which have slight additions of color-causing elements in some specimens that cause it to be a different color. The amount of iron present determines the intensity of the color. Certain minerals exhibit a color change when exposed to light, heat, or radiation. Realgar transforms into orpiment, and orpiment crumbles into a light-yellow powder if exposed to light. Some minerals, such as proustite and vivianite, darken upon prolonged exposure to light, whereas other minerals, such as kunzite fade.
Some minerals undergo color changes when put under intense heat.
This method is sometimes used to artificially enhance the color of some gemstones. Some varieties of topaz , beryl , and corundum are heat treated to produce deep colored gemstones from rather dull ones. Radioactivity can also have an effect, as is the cause of smoky quartz . Iron usually causes a mineral to exhibit a dark red or brown color, and manganese is responsible for the coloring of many pink minerals. Some minerals, such as cassiterite and zincite, have chemical structures that would cause them to be colorless, but due to impurities they are never found colorless.