The Often Deceptive Property of Color in Minerals: Quartz
(from Trivial Pursuits, Albuquerque Gem and Mineral Club)
Color is one of the most obvious features of many minerals. In fact, it may be the most obvious of all. Haven’t you ever found yourself looking at a display of gems or minerals in a closed case and when you want to indicate a particular specimen to someone else, you say, "Hey, look at that bright green (or red, yellow, brown, blue, orange, purple, puce, mauve, taupe, etc.) one over there. The one to the right of the red (etc.) guy." It seems to be instinctive (and it may well be) that we often notice color before the other (usually more reliable) characteristics of minerals and other objects. This is not all fine and good.
Most pure minerals are colorless, clear or white, which means that the other colors we see are due to impurities. (And dependence on color may make it more difficult for students to identify the mineral.) Quartz is one of the best examples of a mineral that is often found in a wide variety of colors, but is clear when pure. These colored varieties are so common, distinctive and beautiful that many have been given names which are familiar to us all. Amethyst is purple; citrine is yellow, orange, and brown; prase is green; smoky quartz (and caringorm and morion) is pale brown to black; rose quartz is pink. Red and blues are also found.
What causes these colors? Well, in many cases, quartz is colored by inclusions of other minerals such as hematite, chlorite, rutile, lepidolite, ajoite, etc. In these instances, you can see the individual grains inside the quartz, and they make the stone cloudy or opaque. But in many other cases, the quartz is clear but colored yellow, purple, etc. It is not colored by mineral impurities, but by elements that replace silicon in the structure of the quartz. Impurities such as small amounts of aluminum or iron.
Now, aluminum, by itself, doesn’t impart color to most things, but if it is irradiated, it produces color centers (F-centers, Farbe centers) and it becomes smoky. It can be naturally irradiated by other minerals in the rock where it grows, or by man using X-Rays, gamma rays, etc. The more aluminum present in the quartz, the darker it gets. Some of this smoky quartz can be turned to a greenish-yellow by heating, but this isn’t universal. Ferric iron is, on the other hand, usually produced (in) yellow quartz. If the iron is in the right place in the quartz structure, the stone can be turned into amethyst by irradiation. Unfortunately, this is rare. But most amethyst can be heated or "burned" into yellow to orange-brown citrine, and this is often done commercially. And if you don’t overburn the stone, you can irradiate it back into amethyst.
(Now class, for your homework assignment: How is Ametrine (1/2 amethyst + ½ citrine) produced?
Republished by permission of Albuquerque Gem and Mineral Club
Paul Hlava, author