Dumortierite

Dumortierite was named after the French scientist, M.E. Dumortier. Dumortierite is best known in its massive form, making an attractive violet and blue decorative gemstone when polished. Reddish brown and red varieties also occur. Prismatic crystals bigger than 1mm are very rare.

Demortierite is also found intergrown with rock crystal (colourless quartz) and is then called dumortierite quartz. This material is generally cut in cabochon or polished to make decorative stones.

Most gem-quality material is found in Nevada (USA). Other localities include France, Madagascar, Norway, Sri Lanka, Canada, Poland, Namibia and Italy.

Dumortierite Properties

Chemical Composition: 
Aluminium Borosilicate - Al7O3(BO3)(SiO4)3
Colors / Varieties: 
Blue, violet, brown, pinkish, bluish green, greenish
Crystal System / Forms: 
Orthorhombic System / Prismatic crystals, often massive, fibrous, granular
Hardness: 
8 - 8.5; massive varieties 7
Specific Gravity: 
3.26 - 3.41
Cleavage / Fracture: 
Perfect 1 directional. Not observed in massive.
Optic Character: 
Anisotropic, D.R.; Biaxial negative. Commonly Opaque.
Lustre: 
Sub-Vitreous
Refractive Index / Birefringence: 
1.686 - 1.723 / 0.037
Pleochroism: 
Strong pleochroism if transparent, otherwise unable to detect.
Magnification: 
Parallel fibrous inclusions, crystals, fingerprints.
U.V. Fluorescence: 
Blue to bluish white in shortwave.
Specific Tests: 
Most gem quality dumortierite is actually dumortierite in quartz and may give R.I. readings for quartz.
Simulants (with separation tests): 
Sugilite (structure), Chalcedony (structure), Fluorite (optic character, R.I., U.V. fluorescence)
Geological Occurrence: 
Aluminous metamorphic rocks.
Sources: 
France, U.S.A. (Arizona), Sri Lanka, Brazil.
Cuts & Uses: 
Generally opaque gemstones are available and are cut into cabochons. Transparent varieties are very rare.