|Crystal system||?-quartz: trigonal trapezohedral class 3 2; ?-quartz: hexagonal 622|
|Space group||Trigonal 32|
|Unit cell||a = 4.9133 Å, c = 5.4053 Å; Z=3|
|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|
|Mohs scale hardness||7 - lower in impure varieties (defining mineral)|
|Lustre||Vitreous - waxy to dull when massive|
|Diaphaneity||Transparent to nearly opaque|
|Specific gravity||2.65; variable 2.59-2.63 in impure varieties|
|Optical properties||Uniaxial (+)|
|Refractive index||n? = 1.543-1.545 |
n? = 1.552-1.554
|Birefringence||+0.009 (B-G interval)|
|Pleochroism||weak, from red-brown to green-brown|
|Melting point||1670 °C (? tridymite) 1713 °C (? cristobalite)|
|Solubility||Insoluble at STP; 1 ppmmass at 400 °C and 500 lb/in2 to 2600 ppmmass at 500 °C and 1500 lb/in2|
|Other characteristics||lattice: hexagonal, Piezoelectric, may be triboluminescent, chiral (hence optically active if not racemic)|
Smoky quartz is a grey, translucent variety of quartz that ranges in clarity from almost complete transparency to an almost-opaque brownish-gray or black crystal. Like other quartz gems, it is a silicon dioxide crystal. The smoky colour results from free silicon formed from the silicon dioxide by natural irradiation.
Morion is a very dark brown to black opaque variety. Morion is the German, Danish, Spanish and Polish synonym for smoky quartz. The name is from a misreading of mormorion in Pliny the Elder. It has a density of 5.4.
Cairngorm is a variety of smoky quartz found in the Cairngorm Mountains of Scotland. 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). The largest known cairngorm crystal is a 23.6 kg (52 lb) specimen kept at Braemar Castle.