DUMORTIERITE: Nevada (~2 inches across); Ogilby, California (~1 inch across). Photo © Joel E. Arem, PhD, FGA. Used with permission.
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.
| Data | Value |
|---|---|
| Name | Dumortierite |
| Varieties | Holtite |
| Alternate Common Names | Holtite |
| Colors | Blue, violet, brown, pinkish, blue-green, greenish. |
| Hardness | 8 - 8.5; massive varieties 7. |
| Fracture | Splintery or uneven |
| Cleavage | Good 1 direction, not observed in massive material |
| 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. |
| Formula | Ab7O3(BO3)(SiO4)3. |
| Crystallography | Orthorhombic. Crystals prismatic and very rare; usually massive, fibrous, granular. |
| Refractive Index | 1.668-1.723 |
| Birefringence | 0.15-0.37 |
| Luminescence | Blue (France) in SW; also blue-white to violet (California) in SW. |
| Luminescence Present | Yes |
| Luminescence Type | Fluorescent, UV-Short |
| Absorption Spectrum | Not diagnostic. |
| Pleochroism | Black/brown/red-brown; also: blue-black/blue/colorless. |
| Optics | a = 1.686; β = 1.722; γ = 1.723. (Brazilian material: a = 1.668-1.673; β = 1.682-1.684; γ = 1.685-1.688). Uniaxial (-), 2V= 13-56°. |
| Optic Sign | Uniaxial - |
| Luster | Vitreous to dull. |
| Specific Gravity | 3.26-3.41 |
| Transparency | Transparent to opaque. |
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.
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