DUMORTIERITE: Nevada (~2 inches across); Ogilby, California (~1 inch across)
Dumortierite is a beautiful and very hard material, eminently suitable for jewelry. The cabochon material is the only generally known form, since faceted stones are so rare. Fibrous inclusions have been noted in the transparent Brazilian stones.
Dumortierite Value
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Dumortierite Information
| Data | Value |
| Name | Dumortierite |
| Alternate Common Names | Holtite |
| Crystallography | Orthorhombic. Crystals prismatic and very rare; usually massive, fibrous, granular. |
| Colors | Blue, violet, brown, pinkish, blue-green, greenish. |
| Luster | Vitreous to dull. |
| Hardness | 8-8.5; massive varieties 7. |
| Density | 3.26-3.41 |
| Birefringence | 0.15-0.37 |
| Cleavage | Good 1 direction, not observed in massive material; fracture splintery or uneven. |
| Stone Sizes | Massive blue and violet material occurs in pieces weighing several pounds. Only a small amount of facetable material has ever been discovered (Brazil, Sri Lanka), and these gems tend to be very small (under 1-2 carats). Only a few faceted dumortierites exist. Arizona dumortierite is actually a quartz-impregnated variety. |
| Luminescence | Blue (France) in SW; also blue-white to violet (California) in SW. |
| Spectral | Not diagnostic. |
| Formula | Ab7O3(BO3)(SiO4)3. |
| Pleochroism | Black/brown/redbrown; also: blue-black/ blue/colorless. |
See also: Holtite.
Optics: a = 1.686; β= 1.722;γ= 1.723.
Brazil: a = 1.668-1.673; β= 1.682-1.684; γ= 1.685-1.688. SG = 3.31-3.35.
Uniaxial (-), 2V= 13-56°.
Occurrence: In aluminous metamorphic rocks; in pegmatites.
Pershing County, Nevada: violet gem material. Arizona.
France; Madagascar; Brazil (Minas Gerais): facetable bluish-green material.
Sri Lanka: transparent, reddish-brown stones.
Comments: Dumortierite is a beautiful and very hard material, eminently suitable for jewelry. The cabochon material is the only generally known form, since faceted stones are so rare. Fibrous inclusions have been noted in the transparent Brazilian stones.
Name: After M. Eugene Dumortier, a paleontologist.
