Dumortierite

Rare mineral of the „silicates and germanates" class

Dumortierit dumortierite

Origin of name: firstly decribed in 1881 by French mineralogist Ferdinand Gonnard, who named it in honour of French paleontologist Eugène Dumortier. Type locality is the Ducarre quarry in Chaponost, Département Rhône, France.

Synonyms and trade names: none

Can be confused with: sodalite, lapis lazuli, lazulite, azurite, chalcedony 

Localities: dumortierite is found on all continents, including Antarctica. Nevertheless, with fewer than 250 known deposits (2025), it is regarded as rare. Important sources are the USA, Canada, Argentina, Brazil, Madagascar, Japan, Norway, the Czech Republic and Austria.

dumortierit dumortieritedumortierit

Handling: dumortierite is sensitive towards acid and has distinct cleavage. Due good hardness (up to 8.5) it still is a good gem stone. 

Gemmological Properties of Dumortierite

Formula:
Al6,5–7[(O,OH)3|BO3|(SiO4)3] idealized
Crystal system:
orthorhombic
Mohs hardness:
7 - 8.5
Specific gravity:
3.21 - 3.41
Refractive index:
birefringent 1.659 - 1.692
Max. Birefringence:
0.027
Pleochroism:
strong, yellowish to bluish white, purplish red, blue
Luminiscence:
none
Lustre:
vitreous, dull
Cleavage:
distinct
Fracture:
uneven
Colour:
blue, greenish bkue, purple, reddish purple, brownish purple