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Agate

SiO₂

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PROPERTIES

All properties

	Agate
Type	Gem
Mineral	Quartz
Formula	SiO₂
Mineral group	Silicates
Crystal system	Trigonal
Mohs hardness	7
Density	2.65 g/cm³
Refractive index	ω 1.544; ε 1.553
Optical class	Uniaxial (+)
Colour	Brown, white, red, grey, pink, black or yellow, typically in bands
Colour cause	Agate forms when silica carried by groundwater is deposited in cavities, in concentric layers around the walls or in horizontal layers building up from the bottom. Its colours come from impurities: groundwaters of different compositions seeping into the cavity leave alternating bands that record changes in water chemistry.
Treatments	Agate is porous and readily accepts dye; most of the spectacularly coloured agates sold in the gemstone trade have been dyed.
Look-alikes	Some petrified wood is called "agatized wood" and taken for agate, but it is chalcedony.
Crystal habit	Commonly a six-sided prism terminated by rhombohedra; also drusy, fibrous, microcrystalline as chalcedony, or massive. More than 500 crystal forms are reported.
Cleavage	Rarely observable; poor on rhombohedral and prismatic directions
Fracture	Conchoidal
Tenacity	Brittle; tough when massive
Luster	Vitreous; waxy to dull in massive varieties
Streak	White
Transparency	Transparent to nearly opaque
Fluorescence	Usually inert; response varies with inclusions. Piezoelectric and pyroelectric, and sometimes triboluminescent.
Identification	Hardness 7 scratches ordinary glass; conchoidal fracture and absent obvious cleavage separate it from feldspar. Calcite is much softer and reacts with dilute acid.
In Tajikistan	Banded chalcedony agate is known from the Adrasman agate occurrence in the Adrasman–Kanimansur ore field and near Dolany village, Sughd region; specimens are held in the Fersman Mineralogical Museum.
Source	Handbook of Mineralogy — Quartz