Cyanite

Common silicate mineral.

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Origin of name: the name cyanite was coined in 1789 by German mineralogist Abraham Gottlob Werner and derives from Greek κύανος (kianos) for blue or cornflower.
Still in use is a second name, disthene, given by French mineralogist René-Just Haüy, which derives from Greek δις σθένος  (dis twice, sthenos force) and alludes to the extreme anisotropy of hardness (see below "Worth knowing").

Synonyms and trade names: cyanite, disthene

Can be confused with: blue cyanite can be confused with sapphire and other blue gems. Identification along optical and physical properties usually is straight forward.

Localities: common worldwide. Mindat.org lists 1479 locations on all continents, even in Greenland and Antarctica (November 2015).

Important localities are Brazil and South Africa.
Gem quality cyanite is found in Nepal.

kyanit cyanite disthen disthenekyanit cyanite disthen disthene
Two cyanites from South Africa
kyanit cyanite disthen disthenekyanit cyanite disthen disthene

Two gemmy cyanites from Nepal

Handling: due to low hardness and perfect cleavage, cyanite is only of limited use as a gemstone. Besides cyanite is very susceptible to heat and should neither be cleaned ultrasonically, nor should it be exposed to galvanic treatments.

Worth knowing: apart from the (sometimes) exraordinary blue colour, cyanites most remarkable property is the extreme anisotropy of Mohs hardness, which can be as low as 4.5 along the c-axis and as high as 7 along other crystallographic axes.
This, as well as the perfect cleavage, makes cyanite rather difficult to cut and polish.

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Gemmological Properties of Cyanite

Formula:
Al2(SiO4)O
Crystal system:
triclinic
Mohs hardness:
4-.5-7
Specific gravity:
3.53-3.67
Refractive index:
doubly refractive 1.712-1.734
Max. Birefringence:
0.016
Dispersion:
weak
Pleochroism:
weak
Luminiscence:
none
Lustre:
vitreous
Cleavage:
perfect
Fracture:
splintery
Colour:
colourless, grey, blue, green, yello, rarely orange, pink or red