"Amethyst" (SiO2) is a violet or purple variety of quartz often used as an ornament. The name comes from the Greek ''a'' (not) and ''methuskein'' ("to intoxicate"), a reference to the belief that the stone protected its owner from drunkenness; the ancient Greeks and Romans wore amethyst and made drinking vessels of it in the belief that it would prevent intoxication.
In the 20th century, the color of amethyst was attributed to the presence of manganese. However, since it is capable of being greatly altered and even discharged by heat, the color was believed by some authorities to be from an organic source. Ferric thiocyanate was suggested, and sulfur was said to have been detected in the mineral.
On exposure to heat, amethyst generally becomes yellow, and much of the citrine, cairngorm, or yellow quartz of jewelry is said to be merely "burnt amethyst". Veins of amethystine quartz are apt to lose their color on the exposed outcrop.
Amethyst is composed of an irregular superposition of alternate lamellae of right-handed and left-handed quartz. It has been shown that this structure may be due to mechanical stresses. As a consequence of this composite form...