Smoky quartz

Smoky quartz is a brownish grey, translucent variety of quartz that ranges in clarity from almost complete transparency to an almost-opaque brownish-gray or black crystals.^[7] The color of smoky quartz is produced when natural radiation, emitted from the surrounding rock, activates color centers around aluminum impurities within the crystalline quartz. ^[8]

Smoky quartz
General
Category	Tectosilicates, quartz variety
Formula	SiO2
Strunz classification	04.DA.05
Dana classification	75.01.03.01
Crystal system	α-quartz: trigonal trapezohedral class 3 2; β-quartz: hexagonal 622[1]
Space group	Trigonal 32
Unit cell	a = 4.9133 Å, c = 5.4053 Å; Z=3
Identification
Colour	Brown to grey, opaque
Crystal habit	6-sided prism ending in 6-sided pyramid (typical), drusy, fine-grained to microcrystalline, massive
Twinning	Common Dauphine law, Brazil law and Japan law
Cleavage	{0110} none[2]
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	7 – lower in impure varieties (defining mineral)
Lustre	Vitreous – waxy to dull when massive
Streak	White
Diaphaneity	Transparent to nearly opaque
Specific gravity	2.65; variable 2.59–2.63 in impure varieties
Optical properties	Uniaxial (+)
Refractive index	nω = 1.543–1.545 nε = 1.552–1.554
Birefringence	+0.009 (B-G interval)
Pleochroism	weak, from red-brown to green-brown
Melting point	1670 °C (β tridymite) 1713 °C (β cristobalite)[1]
Solubility	Insoluble at STP; 1 ppmmass at 400 °C and 500 lb/in2 to 2600 ppmmass at 500 °C and 1500 lb/in2[1]
Other characteristics	lattice: hexagonal, Piezoelectric, may be triboluminescent, chiral (hence optically active if not racemic)
References	[3][4][5][6]

Varieties

Morion is a very dark brown to black opaque variety. Morion is the German, Danish, Spanish and Polish synonym for smoky quartz.^[9] The name is from a misreading of mormorion in Pliny the Elder.^[10]

Cairngorm is a variety of smoky quartz found in the Cairngorm Mountains of Scotland.^[11] It usually has a smoky yellow-brown colour, though some specimens are greyish-brown. It is used in Scottish jewellery and as a decoration on kilt pins and the handles of sgianan-dubha (anglicised: sgian-dubhs or skean dhu).^[12] The largest known cairngorm crystal is a 23.6 kg (52 lb) specimen kept at Braemar Castle.^{[citation needed]}

Uses

Smoky quartz is common and was not historically important, but in recent times it has become a popular gemstone, especially for jewelry.^[13]

Sunglasses, in the form of flat panes of smoky quartz, were used in China in the 12th century.^[14]

See also

• List of minerals