Bismutotantalite Value, Price, and Jewelry Information

“Bismutotantalite,” Madagascar, 0.5 cts. Photo © Joel E. Arem, PhD, FGA. Used with permission.

Bismutotantalite

Extremely rare as a cut gem, even in very complete collections. Many of the minerals in the tantalite group have been faceted; bismutotantalite is perhaps the rarest of them all. The color is attractive, but low hardness and good cleavage make use in jewelry unadvisable.

Bismutotantalite Information

Data Value
Name Bismutotantalite
Colors Light brown to black
Crystallography Orthorhombic. Crystals sometimes large; massive; stream pebbles.
Refractive Index 2.388-2.428
Luster Adamantine to submetallic.
Hardness 5
Fracture Subconchoidal
Specific Gravity 8.84 (Brazil)
Birefringence 0.040
Cleavage Perfect 1 direction
Dispersion No data
Stone Sizes Cut gems always small, less than 5 carats
Luminescence None
Luminescence Present No
Transparency Opaque, transparent in very thin fragments.
Absorption Spectrum Not diagnostic.
Formula

(Bi, Sb) (Ta, Nb)O4

Pleochroism

No data

Optics

α = 2.388; β = 2.403; γ = 2.428. Biaxial (+), 2V = 80°.

Optic Sign Biaxial +

STREAKYellow-brown to black

OPTICS: α = 2.388; β = 2.403; γ = 2.428

Biaxial (+), 2V = 80°

OCCURRENCE: Pegmatites.

Gamba Hill, Uganda; Acari, Brazil.

COMMENTS: Extremely rare as a cut gem, even in very complete collections. Many of the minerals in the tantalite group have been faceted; bismutotantalite is perhaps the rarest of them all. The color is attractive, but low hardness and good cleavage make use in jewelry unadvisable.

NAME: On account of the composition.

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