The name amethyst comes from the Greek "a-µe??stos" which means "not drunk". Roman emperors had cups/goblets made form amethyst in the belief that the cup would counteract any poison and keep them from getting drunk. The ancients thought amethyst to be different from quartz. They thought quartz was water that had been frozen so hard that it would never melt. Amethyst is a variety of quartz and has basically the same chemical composition, atomic structure and physical properties of quartz, (hardness 7, specific gravity of 2.65 g/cc etc.) Books on geology and mineralogy usually list it under quartz. The bible mentions amethyst as one of the twelve stones in the breastplate of the High Priest and thus represented one of the 12 tribes of Israel.

When the variety of quartz called amethyst is being formed, the water supplying the nutrients for the growing crystal carry some dissolved iron in its ferric (Fe+3) state. This is incorporated into the structure of the growing crystal. When the ferric iron is incorporated into the structure of the quartz crystal it forms "color centers". Natural ionizing radiation in the ground then strips another electron from the ferric iron atom, this activates the color center which gives the amethyst its color. That means that the color center absorbs all the colors except those in the purple/violet region which are transmitted and reflected to our eyes. The intensity and color of the amethyst depends on how much iron is in the quartz crystal, and where it is located in the atomic structure of the quartz, and the possible presence of aluminum which may impart a slightly smoky color to the amethyst. Well, that's the general idea at any rate. Want to know more? No? Well, on the off chance you do, refer to A. Cohen & F. Hassan's excellent article in the 1974 Canadian Mineralogist, Vol.59, pp 719-728.