... cavities of mineral-veins and in granitic rocks, or as a lining in agate geodes. A huge geode, or "amethyst-grotto," from near Santa Cruz in southern Brazil was ...
Amethyst
From Wikipedia, the free encyclopedia
For other uses, see Amethyst (disambiguation).
Quartz
General
Category Mineral variety
Chemical formula Silica (silicon dioxide, SiO2)
Identification
Color purple
Crystal habit 6-sided prism ending in 6-sided pyramid (typical)
Crystal system rhombohedral class 32
Twinning Dauphine law, Brazil law and Japan law
Cleavage None
Fracture Conchoidal
Mohs Scale hardness 7 - lower in impure varieties
Luster Vitreous/glossy
Refractive index nω = 1.543 - 1.553 nε = 1.552 - 1.554
Optical Properties Uniaxial (+) (Positive)
Birefringence +0.009 (B-G interval)
Pleochroism None
Streak White
Specific gravity 2.65 constant; variable in impure varieties
Melting point 1650 (±75) °C
Solubility H2O insoluble
Diaphaneity Transparent to translucent
Other Characteristics Piezoelectric
Amethyst is a purple variety of quartz often used as an ornament. The name comes from the Greek 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.
[edit]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.[1] Further study has shown a complex interplay of iron and aluminium is responsible for the color.[2]
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 [citation needed].
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[3] 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.[4]
