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
Crystallography Orthorhombic. Crystals sometimes large; massive; stream pebbles.
Refractive Index 2.388-2.428
Colors Light brown to black
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.

Ready to learn how to identify gems on your own?

Join our mailing list below to download a FREE gem ID checklist tutorial. See what’s inside…

• Discover the 17 practical steps to gemstone identification (even if you’re just getting started with gemology)

• Learn how you can use specific tools to gather data, make observations & arrive at an accurate ID

• Explore a range of gemological tests… not only will you get familiar with the process but also time-saving shortcuts!