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Amethyst

This page kindly sponsored by Kenzay Gemstone Jewelry
Formula:
SiO2
Colour:
Violet - purple
Lustre:
Vitreous
Name:
From Greek "a-methystos", meaning not drunk.
A variety of Quartz

A violet to purple variety of quartz that owes its color to gamma irradiation (Berthelot, 1906) and the presence of traces of iron built into its crystal lattice (Holden, 1925). The irradiation causes the iron Fe(+3) atoms that replace Si in the lattice to lose an electron and form a [FeO4]0 color center (Lehmann and Moore, 1966).

Changes in color of amethyst by heat treatment and UV radiation. Details on treatment in photo caption.

The color in amethyst from most localities is unevenly distributed in the individual crystals. In amethyst geodes it is often most intense in the growth zones under the rhombohedral faces (at the tips). Occasionally the color is deeper under either the r or z rhombohedral faces, giving the crystal a pinwheel appearence when viewed from the top. In prismatic crystals the color may appear in phantom-like thin layers, while in scepters and skeleton quartz the color is often concentrated along the edges, and accompanied by smoky zones. Despite the intense color, the content of iron occupying Si positions in amethyst is rather low, in the 10-100 ppm range (Dennen and Puckett, 1972).

When heated to more than about 300-400°C, amethyst loses its violet color and often turns yellow, orange or brown, and then resembles the quartz variety citrine, but depending on the locality and the temperature during the heat treatment it may also turn colorless or - rarely - green (Rose and Lietz, 1954; Neumann and Schmetzer, 1984).

Irradiation with UV light will also destroy the color centers, and accordingly prolonged exposure to sunlight will slowly fade amethyst (Currier, 1985). The photo to the right shows the effects of heat (bottom left and right) and UV irradiation (top right) on the color of a specimen from Uruguay.

Amethyst is pleochroic (Haidinger, 1847; Pancharatnam, 1954; Raman, 1954): when the polarization of the light is changed from parallel to the c-axis to perpendicular to the c-axis, amethyst changes its color from blue-violet to purple. The strength of the effect varies to a considerable degree, and changes in the hue depending on the direction of the transmitted light may be observable with the naked eye, in particular in crystals with a zonar development of color, which may even show sky-blue tones.

Amethyst crystals do not get very large, crystals longer than 30cm are very rare.
It is found in various forms and shapes, the most common growth forms are:
1. Druzy crystal aggregates that outline cavities; the crystals are usually short-prismatic and often lack prism faces. Most common in volcanic rocks, but also in hydrothermal veins, and even in cavities in sedimentary rocks;
2. Scepters (late syntaxial overgrowth) on other color varieties of quartz, in particular in high- to medium-temperature environments like alpine-type fissures and pegmatites;
3. Split-growth crystals ("artichoke quartz") in hydrothermal veins in ore deposits, but also in volcanic rocks.
4. As individual well-formed crystals in small cavities and fissures, in particular in volcanic rocks.
5. As hydrothermal vein filling, often with several growth phases with variable color that cause a banding pattern.

Druzy amethyst
Amethyst scepter
Amethyst with split growth
Prismatic well-developed crystals
Vein quartz made of "Chevron amethyst"
Druzy amethyst
Amethyst scepter
Amethyst with split growth
Prismatic well-developed crystals
Vein quartz made of "Chevron amethyst"
Druzy amethyst
Amethyst scepter
Amethyst with split growth
Prismatic well-developed crystals
Vein quartz made of "Chevron amethyst"


Despite the large number of different shapes amethyst shows, certain types of quartz have never been found as amethyst: Macromosaic quartz, frequently found as smoky quartz or citrine, apparently never forms amethyst. Amethyst gwindels are unknown (only gwindels with amethyst overgrowth have been found). Amethyst does also not seem to form needle-like crystals.

Polysynthetic twinning in amethyst

In general, only the basic crystallographic forms of quartz, the prism and the positive and negative rhombohedra are found on amethyst crystals. Druzy amethyst may lack prism faces. Crystals with additional faces are known only from very few localities.

Amethyst often contains zones of polysynthetic Brazil law twinning, that is, it is composed of alternating layers of right- and left-handed twin domains, typically under the positive rhombohedral faces ("r faces" {1 0 -1 1}; Brewster 1823; McLaren and Pitkethly, 1982; Taijing and Sunagawa, 1990). Usually this cannot be recognized on the crystal's surface unless it is etched, but some amethysts from certain localities show a "fingerprint" pattern on the rhombohedral faces that reflects the geometry of the left- and right-handed domains, like the specimen from Uruguay shown to the right.


Visit gemdat.org for gemological information about Amethyst.

Classification of Amethyst

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Occurrences of Amethyst

Geological Setting:
Very common, in many different environments. The commercially most important occurrences are in volcanic rocks, where amethyst crystals outline former gas cavities. In low and medium temperature hydrothermal veins associated with iron ores. As late overgrowth ("scepter quartz") on quartz in pegmatite and alpine-type environments.

Physical Properties of Amethyst

Vitreous
Colour:
Violet - purple
Comment:
pleochroic: blue-violet to purple

Chemical Properties of Amethyst

Formula:
SiO2
All elements listed in formula:

Relationship of Amethyst to other Species

Other Names for Amethyst

Name in Other Languages:
Basque:Amatista
Bulgarian:Аметист
Catalan:Ametista
Czech:Ametyst
Danish:Ametyst
Dutch:Amethist
Esperanto:Ametisto
Estonian:Ametüst
Finnish:Ametisti
French:Améthyste
Hungarian:Ametiszt
Irish Gaelic:Aimitis
Italian:Ametista
Kazakh (Cyrillic Script):Аметист
Latvian:Ametists
Lithuanian:Ametistas
Norwegian (Bokmål):Ametyst
Polish:Ametyst
Portuguese:Ametista
Romanian:Ametist
Samogitian:Ametėsts
Serbian (Cyrillic Script):Аметист
Simplified Chinese:紫晶
紫水晶
Slovak:Ametyst
Slovenian:Ametist
Spanish:Amatista
Swedish:Ametist
Turkish:Ametist
Ukrainian:Аметист
Wolof:Ametist

Other Information

Health Risks:
Quartz is usually quite harmless unless broken or powdered. Broken crystals and masses may have razor-sharp edges that can easily cut skin and flesh. Handle with care. Do not grind dry since long-term exposure to finely ground powder may lead to silicosis.

References for Amethyst

Reference List:
Brewster, D. (1823) On circular polarization, as exhibited in the optical structure of the amethyst, with remarks on the distribution of the colouring matter in that mineral. Transactions of the Royal Society of Edinburgh: 9: 139-152.

Haidinger, W. (1847) Ueber den Pleochroismus des Amethysts. Annalen der Physik: 146: 531-544.

Berthelot, M. (1906) Synthèse du quartz améthyste; recherches sur la teinture naturelle ou artificielle de quelques pierres précieuses sou les influences radioactives. Comptes Rendus de l'Académie des Sciences: 143: 477-488. [Synthesis of amethyst quartz. Researches on the color, natural or artificial, of some precious stones under radioactive influences.]

Holden, E.F. (1925) The cause of color in smoky quartz and amethyst. American Mineralogist: 9: 203-252.

Raman, C.V., Banerji, K. (1925) The optical properties of amethyst. Transactions of the Optical Society: 26: 289-292.

Mügge, O. (1932) Zwillingsbau amethystfarbener Quarze mit Rücksicht auf ihre Entstehungstemperatur. Zeitschrift für Kristallographie: 83: 460-484.

Frondel, C. (1934) Origin of the segmental coloration of amethyst and smoky quartz. American Museum Novitates: 758: 1-15.

Bappu, M.K.V. (1952) Spectroscopic study of amethyst quartz in the visible region. Indian Journal of Physics: 26: 1-14.

Bappu, M.K.V. (1952) Spectroscopic study of amethyst quartz in the ultraviolet and infrared regions. Indian Journal of Physics: 26: 385-392.

Pancharatnam, S. (1954) On the pleochroism of amethyst quartz and its absorption spectra. Proceedings of the Indian Academy of Sciences - Section A: 40:196-210.

Raman, C.V, Jayaraman, A. (1954) The structure of amethyst quartz and the origin of its pleochroism. Proceedings of the Indian Academy of Sciences, Section A: 40: 189-195.

Raman, C.V, Jayaraman, A. (1954) The structure of amethyst quartz and its genesis in nature. Proceedings of the Indian Academy of Sciences, Section A: 40: 221-229.

Rose, H., Lietz, J. (1954) Ein grün verfärbbarer Amethyst. Naturwissenschaften: 41: 448.

Cohen, A.J. (1956) Color centers in the α-quartz called amethyst. American Mineralogist: 41: 874-891.

Lietz, J., Münchberg, W. (1957) The coloring of amethyst. II. The effect of ultraviolet and gamma-rays on the coloring. Neues Jahrbuch für Mineralogie, Monatshefte: 1957: 25-33.

Pancharatnam, S. (1958) The optic interference figures of amethystine quartz. Part I. Proceedings of the Indian Academy of Sciences, Section A: 47: 201-209.

Pancharatnam, S. (1958) The optic interference figures of amethystine quartz. Part II. Proceedings of the Indian Academy of Sciences, Section A: 47: 220-229.

Chudoba, K.F. (1961) The significance of the color of amethyst. Aufschluss: 12: 233-248 (in German).

Chudoba, K.F. (1962) Some relations between the causes of amethyst, smoky quartz, and citrine colors as given by modern science. Mineralogicheskii Sbornik (Lvov): 16: 91-105.

Lehmann, G., Moore, W.J. (1966) Color center in amethyst quartz. Science: 152: 1061-1062.

McLaren, A.C., Phakey, P.P. (1966) Electron microscope study of Brazil twin boundaries in amethyst. Physica Status Solidi: 13: 413-422.

Lehmann, G. (1967) Iron color center as the cause of amethyst color. Zeitschrift für Naturforschung, Teil A: Astrophysik, Physik und Physikalische Chemie: 22: 2080-2085.

Dennen, W.H., Puckett, A.M. (1972) On the chemistry and color of amethyst. The Canadian Mineralogist: 11: 448-456.

Lehmann, G., Bambauer, H.U. (1973) Quartz crystals and their colors. Angewandte Chemie International Edition: 12: 283-290.

Hassan, F., Cohen, A.J. (1974) Biaxial Color Centers in Amethyst Quartz. American Mineralogist 59, 709-718.

Cohen, A.J., Hassan, F. (1974) Ferrous and ferric ions in synthetic alpha-quartz and natural amethyst. American Mineralogist: 59: 719-728.

McLaren, A.C., Pitkethly, D.R. (1982) The twinning microstructure and growth of amethyst quartz. Physics and Chemistry of Minerals: 8: 128-135.

Neumann, E., Schmetzer, K. (1984) Mechanism of thermal-conversion of color and color-centers by heat-treatment of amethyst. Neues Jahrbuch für Mineralogie Monatshefte: 6: 272-282.

Balitskii, V.S., Balitskaya, O.V. (1986) The amethyst-citrine dichromatism in quartz and its origin. Physics and Chemistry of Minerals: 13: 415-421.

Cohen, A.J. (1985) Amethyst color in quartz, the result of radiation protection involving iron. American Mineralogist: 70: 1180-1185.

Currier, R.H. (1985) Natural fading of amethyst. Gems & Gemology: 21(2): 115-116.

Adekeye, J.J.D, Cohen, A.J. (1986) Correlation of Fe4+ optical anisotropy, Brazil twinning and channels in the basal plane of amethyst quartz. Applied Geochemistry: 1: 153-160.

Baran, Z., Godwod, K., Warminski, T. (1987) X-ray study of Brazil twins in natural amethyst. Physica Status Solidi: 101: 9-24.

Kostov, R. (1987) A discussion on the color of amethyst. Priroda (Sofia): 36(2): 10-15.

Schmetzer, K. (1987) Microscopic observation of twinning microstructure in natural amethyst. Neues Jahrbuch für Mineralogie, Monatshefte: 8-15.

Epstein, D.S. (1988) Amethyst mining in Brazil. Gems & Gemmology: 44: 214-228.

Martin, B., Röller, K., Flörke, O.W. (1988) Über die polysynthetische Rechts-Links-Verzwilligung von Amethyst-Quarz und der mikrokristallinen Quarzspezies Chalzedon und Quarzin. Zeitschrift für Kristallographie: 182: 179-180.

Cohen, A.J. (1989): New data on the cause of smoky and amethystine color in quartz. Mineralogical Record: 20: 365-367.

Stegger, P., Lehmann, G. (1989) The structures of three centers of trivalent iron in α-quartz. Physics and Chemistry of Minerals: 16: 401-407.

Taijing, L., Sunagawa, I. (1990) Structure of Brazil twin boundaries in amethyst showing brewster fringes. Physics and Chemistry of Minerals: 17: 207-211.

Choh, S.H., Jo, S.H., Song, K.J., Park, I.W., Han, C.S. (1993) Thermal stability of iron(3+) centers in natural and synthetic amethyst. Bulletin of Magnetic Resonance: 15: 35-40.

Vasconcelos, P., Wenk, H.R., Rossman, G.R. (1994) The Anahí ametrine mine, Bolivia. Gems and Gemology: 30: 4-23.

Rossman, G.R. (1994) The colored varieties of the silica minerals. In: P.J. Haney, ed., Reviews in Mineralogy 29, 433-468.

Lieber, W. (1994) Amethyst - Geschichte, Eigenschaften, Fundorte. Christian Weise Verlag, München.

Balitsky, V.S., Lu, T., Rossman, G.R., Makhina, I.B., Mar'in, A.A., Shigley, J.E., Elen, S., Dorogovin, B.A. (1999) Russian synthetic ametrine. Gems and Gemology: 35: 122-134.

Dedushenko, S.K., Makhina, I.B., Mar'in, A.A., Mukhanov, V.A., Perfiliev, Y.D. (2004) What oxidation state of iron determines the amethyst colour? Hyperfine Interactions: 156/157: 417-422.

Balzer, R. (2008) Amethyst Uruguay. 304 pp (in German and English).

Tumuklu, A., Gumus, H., Sen, S. (2008) Role of trace elements in natural amethysts in colouring. Asian Journal of Chemistry: 20: 4138-4140.

Lameiras, F.S., Nunes, E.H.M., Vasconcelos, W.L. (2009) Infrared and chemical characterization of natural amethysts and prasiolites colored by irradiation. Materials Research: 12: 315-320.

Russo, F.T. (2011) Investigation of optical, electrical and structural properties in Amethyst. Dissertation (Master in Science and Technology of Materials), UNESP, FC, Bauru, 113pp, (in Portugese).

Gilg, H.A., Liebtrau, S., Staebler, G.A., Wilson, T. (editors) (2012) Amethyst: Uncommon Vintage ExtraLapis English No.16, Lithographie. Denver 124pp.

Internet Links for Amethyst

Specimens:
The following Amethyst specimens are currently listed for sale on minfind.com.

Localities for Amethyst

map shows a selection of localities that have latitude and longitude coordinates recorded. Click on the symbol to view information about a locality. The symbol next to localities in the list can be used to jump to that position on the map.
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