Amethyst
Amethyst is a purple variety of quartz often used as an ornamental stone in jewellery. more...
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The name comes from the Hellènic a- ("not") and methustos ("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.
Chemistry
Amethyst is the purple variety of quartz, its chemical formula is SiO2.
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.
More recent work has shown that amethyst's coloration is due to ferric iron impurities. Further study has shown a complex interplay of iron and aluminium is responsible for the color.
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.
Synthetic amethyst is made to imitate the best quality amethyst. Its chemical and physical properties are so similar to that of natural amethyst that it cannot be differentiated with absolute certainty without advanced gemological testing (which is often cost-prohibitive). There is one test (which is not 100 percent certain) based on "Brazil law twinning" (a form of quartz twinning where right and left hand quartz structures are combined in a single crystal which can be used to identify synthetic amethyst rather easily. In theory however it is possible to create this material synthetically as well, but this type is not available in large quantities in the market.
Composition
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 formation, amethyst is apt to break with a rippled fracture or to show "thumb markings", and the intersection of two sets of curved ripples may produce on the fractured surface a pattern something like that of "engine turning". Some mineralogists, following Sir David Brewster, apply the name of amethyst to all quartz which exhibits this structure, regardless of color.
Read more at Wikipedia.org
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