Provenite

About Amethyst

SiO₂

The violet to purple variety of quartz. The colour is caused by a trace of iron (typically 10-100 ppm) replacing silicon in the lattice, which forms an [FeO4]0 colour centre after natural gamma irradiation; Fe, not Si or O, is the colour agent and so is not listed under Elements. It is pleochroic (blue-violet to purple depending on the direction relative to the c-axis). Colour distribution is frequently patchy, with phantom layers and concentration along crystal edges. Synthetic amethyst exists; growth zoning and Brazil-law twinning in natural crystals help to separate it from synthetic material.

Show 15 localities on the map

Photos of Amethyst

Amethyst crystals up to 4.5 cm in a calcite-lined cavity on matrix.
Amethyst · Goboboseb Mountains
Amethyst crystals up to 4.5 cm in a calcite-lined cavity on matrix.
Photo: Robert M. Lavinsky · CC BY-SA 3.0 · Source
Amethyst crystals lying across a rounded matrix, many with both ends developed, from Piedra Parada, Veracruz, Mexico.
Amethyst · Piedra Parada (Tatatila)
Amethyst crystals lying across a rounded matrix, many with both ends developed, from Piedra Parada, Veracruz, Mexico.
Photo: Robert M. Lavinsky · CC BY-SA 3.0 · Source
Small cluster of amethyst crystals with colour zoning, about 5 cm, from Amatitlan, Guerrero, Mexico.
Amethyst · Amatitlan amethyst veins (Eduardo Neri)
Small cluster of amethyst crystals with colour zoning, about 5 cm, from Amatitlan, Guerrero, Mexico.
Photo: Robert M. Lavinsky · CC BY-SA 3.0 · Source
Cluster of intergrown bright purple amethyst crystals with glassy luster from Jacksons Crossroads, Georgia.
Amethyst · Jackson Crossroads Amethyst Mine
Cluster of intergrown bright purple amethyst crystals with glassy luster from Jacksons Crossroads, Georgia.
Photo: Robert M. Lavinsky · CC BY-SA 3.0 · Source
Sharp amethyst crystals on matrix, the largest 1.7 cm, from the Capurru Quarry near Osilo, Sardinia.
Amethyst · Capurru Quarry (Osilo)
Sharp amethyst crystals on matrix, the largest 1.7 cm, from the Capurru Quarry near Osilo, Sardinia.
Photo: Robert M. Lavinsky · CC BY-SA 3.0 · Source
Amethyst stalactite about 18 cm long with crystals around the outside and a polished base showing growth rings, from Artigas, Uruguay.
Amethyst · Los Catalanes Gemological District
Amethyst stalactite about 18 cm long with crystals around the outside and a polished base showing growth rings, from Artigas, Uruguay.
Photo: Robert M. Lavinsky · CC BY-SA 3.0 · Source
Cabinet specimen of glassy purple amethyst crystals, the largest 3.1 cm across, from the Diamond Willow Mine near Pearl, Ontario.
Amethyst · Diamond Willow Mine (Pearl Station)
Cabinet specimen of glassy purple amethyst crystals, the largest 3.1 cm across, from the Diamond Willow Mine near Pearl, Ontario.
Photo: Robert M. Lavinsky · CC BY-SA 3.0 · Source
Large single amethyst crystal about 16 cm long with a gemmy purple top, from the Anahi mine, Bolivia.
Amethyst · Anahi Mine
Large single amethyst crystal about 16 cm long with a gemmy purple top, from the Anahi mine, Bolivia.
Photo: Robert M. Lavinsky · CC BY-SA 3.0 · Source

Where Amethyst is found (15)

Bolivia

Brazil

Canada

Italy

Mexico

Namibia

Romania

Russia

United States

Uruguay

Zambia

Amethyst on the same specimen with other minerals

Same specimen Amethyst growing on Galena at Naica

A Naica specimen shows gemmy purple amethyst crystals to about 3 cm rising from a bed of bright metallic galena crystals.

Deep purple amethyst on a shiny lead-grey base is a striking and unusual colour pairing.

Sources: SRC-158 SRC-159

Same specimen Amethyst growing on Pyrite at Cavnic

A large Cavnic specimen carries pastel purple, lightly frosted amethyst clusters to about 5 cm on a plate covered with brass-yellow striated pyrite crystals.

Amethyst of this size is described as rare from the Cavnic mines, and the golden pyrite base adds contrast.

Sources: SRC-160 SRC-161

Same specimen Amethyst growing on Chalcopyrite at Cavnic

On the same type of Cavnic plate, iridescent brassy chalcopyrite crystals occur on the matrix together with the pyrite, and the amethyst clusters rise from this sulfide-covered matrix.

The iridescent chalcopyrite adds a second metallic colour beneath the purple quartz.

Sources: SRC-160 SRC-161

Identity

Variety of
Quartz
Strunz
4.DA · 4 Oxides and hydroxides
IMA status
Variety
Name origin
From Greek 'a-methystos': not drunk.
Elements
Si, O

Physical properties

Color
Violet to purple; colour often uneven and strongest in growth zones under the rhombohedral faces
Streak
White
Luster
Vitreous luster
Transparency
Variable
Mohs hardness
7
Density
2.65–2.66 g/cm³
Cleavage / fracture
Cleavage rarely observable and poor; fracture conchoidal; brittle

Crystallography

Crystal system
Trigonal
Space group
P3121 or P3221 (enantiomorphic pair)
Structure
Framework of corner-sharing SiO4 tetrahedra (tectosilicate); cell a = 4.9135 Å, c = 5.4050 Å, Z = 3.
Typical habit
Druzy aggregates, scepters, split-growth 'artichoke' quartz, single well-formed prismatic crystals, and banded vein fillings; crystals longer than 30 cm are very rare
Twinning
Polysynthetic Brazil-law twinning is common, typically beneath the positive rhombohedral (r) faces; other quartz twin laws (Dauphine, Japan) also occur
Optical data
Uniaxial (+); omega = 1.544, epsilon = 1.553; birefringence about 0.009

Formation and use

Formation
Grows in open cavities: as linings of geodes in flood basalt (low-temperature, groundwater-related growth at roughly 15-60 C in Uruguay), in cavities and fractures in brecciated quartzite, in vugs along faults and quartz veins in granitic rock, and in banded hydrothermal vein fillings.
Uses
Faceted gemstone and ornamental material; geodes and crystal groups are collected as specimens.
Stability and care
Heating above about 300-400 C removes the violet colour and usually gives yellow, orange or brown material that resembles citrine (colourless or, rarely, green material is also possible). Ultraviolet light destroys the colour centres, so prolonged sunlight slowly fades amethyst. Keep specimens out of strong light. Broken edges are sharp, and dry grinding produces hazardous quartz dust.

Related minerals

Related minerals
Quartzcitrineametrine
Polymorphs
Cristobalitetridymitecoesitestishovite
Can be confused with
Synthetic amethyst (hydrothermally grown)citrine (including heated amethyst)

Sources

  1. SRC-084 · Book
    Handbook of Mineralogy - QuartzAnthony, J.W.; Bideaux, R.A.; Bladh, K.W.; Nichols, M.C. · 2001 · Mineralogical Society of America / Mineral Data Publishing
  2. SRC-085 · Database
    Quartz (Mindat.org mineral page)Mindat.org / Hudson Institute of Mineralogy
  3. SRC-099 · Database
    Amethyst: Mineral information, data and localitiesMindat.org
  4. SRC-101 · Website
    Citrine DescriptionGemological Institute of America (GIA)
  5. SRC-107 · Article
    Amethyst from Four Peaks, ArizonaLowell, G.R. (author as given in the URL; not verified) · 2004 · Gems & Gemology (Winter 2004)

Draft data from the Provenite workbook. Check entries against the cited sources before relying on them. Photos are open-licence images from Wikimedia Commons; the author and licence are given with each photo.