Tantalite is too dark to be of use as a faceted gem but is sometimes cut as a collector curiosity, either faceted or in cabochons. These could be of any desired size.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
|Crystallography||Orthorhombic. Crystals tabular prismatic, in aggregates, massive, compact.|
|Colors||Black, brownish black, reddish brown; may tarnish iridescent.|
|Luster||Submetallic to vitreous.|
|Density||8.2; decreases with Ta content (columbite, 5.2).|
|Cleavage||Distinct 1 direction. Fracture uneven. Brittle.|
|Stone Sizes||Very large crystals weighing many pounds have been found. The material is usually dark colored, opaque, and of interest only for cabochons.|
TANTALITE Series to Columbite: (Fe,Mn)(Nb,Ta)2O6.
Streak: Black, brownish black, reddish brown.
Optics: a= 2.26; β= 2.30-2.40; γ=2.43.
Biaxial (+), 2VLarge. Usually opaque, indices measured on powders or thin splinters
Spectral: Not diagnostic.
Occurrence: In granite pegmatites.
Colorado; Wyoming; New England.
Canada; Brazil; Madagascar; France; Sweden: Finland; USSR; Zimbabwe; Western Australia.
South Dakota: various localities.
California: various localities.
Comments: Tantalite is too dark to be of use as a faceted gem but is sometimes cut as a collector curiosity, either faceted or in cabochons. These could be of any desired size.
Name: After the mythical character Tantalus, because it is difficult to dissolve the mineral in acids prior to analysis.