Crystalline quartz in shades of purple, lilac or mauve is called amethyst, a stone traditionally worn to guard against drunkenness, and to instil a sober and serious mind. Amethyst is dichroic, showing a bluish or reddish purple tinge when viewed from different angles. Usually faceted as a mixed or step-cut, amethyst has distinctive inclusions which look like tiger-stripes, thumb-prints, or feathers. Some amethyst is heat-treated to change the color to yellow, producing citrine. Crystals that are part citrine and part amethyst are called ametrine.
Amethyst is found in alluvial deposits or in geodes. Some of the largest geodes containing amethyst are in Brazil. Amethyst from the Urals (Russia) has a reddish tinge; Canadian amethyst is violet. Other localities include Sri Lanka, India, Uruguay, Madagascar, USA, Germany, Australia, Namibia and Zambia.
Poor quality material is often tumbled to make beads. If a stone is pale it may be set in a closed setting or have foil placed behind it to enhance the colour. Amethyst has been imitated by glass and synthetic corundum.
Chemical Composition:Silica (Silicon Dioxide) SiO2
Colors / Varieties:Violet
Crystal System / Forms:Trigonal System
Specific Gravity:2.63 - 2.68
Cleavage / Fracture:No distinct cleavage / conchoidal to uneven fracture
Optic Character:Anisotropic, D.R.; Uniaxial positive. May exhibit a bull's eye optic figure (quartz rotates the plane of polarisation parallel to the c-axis).
Refractive Index / Birefringence:1.544 - 1.553 / 0.009. Range: 1.535 - 1.560
Pleochroism:Purple / Pinkish Purple
Magnification:Liquid and two phase, color zoning, negative crystals, zebra-stripe finger prints (structural), crystal inclusions, green fuchsite mica flakes / platelets in aventurine quartz, rutile / tourmaline needles in sagenitic quartz. Brazil law twinning is seen in natural quartz.
Cause of Color:Color center caused by natural or artificial irradiation of Iron (Fe2+ / Fe3+) bearing quartz.
- Piezoelectric: develops an electric charge when pressure is applied.
- Pyroelectricity: develops an electrical charge when heated.
- Diasterism in star variety (star visible in reflected and transmitted light).
- Identification: type of twinning, seed plate, breadcrumb inclusions, Raman / infra-red spectroscopy.
Simulants (with separation tests):Scapolite (optic figure/sign, U.V. fluorescence, inclusions, doubling), synthetic amethyst (inclusions, infra-red spectroscopy), glass (optic character), iolite (optic figure, pleochroism), natural spinel (optic character).
Geological Occurrence:Very widespread, in pegmatite.
Sources:Brazil, India, South Africa, Namibia, Madagascar, Zambia, Sri Lanka.
Cuts & Uses:Facetted cuts, cabochon, carvings, beads, etc.
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