Crystal system Orthorhombic
Transparency Transparent to Opaque
Luster Adamantine to resinous
Fracture Uneven
Cleavage Perfect pinacoidal
Specific Gravity 7.50
Hardness 5
Optical Character Biaxial+ Double Refractive
Refractive index 2.37 - 2.45
Birefringence 0.083
Dispersion strong
Pleochroism Trichroic; strong in deep colors
Chemical Formula Sb(Ta,Nb)O4
Comments Attacked by hydrofluoric acid
Streak white to grayish

Stibiotantalite (stib-ee-OH-tan-tah-lite) is a rare pegmatite mineral, an oxide of antimony, tantalum and niobium with the chemical formula Sb(Ta,Nb)O4.

This mineral ranges from colorless to yellow, reddish brown and brown. The finest crystals, which are very rare, are thin, tubular striated individual crystals, transparent to translucent, of a dark brown to brownish-yellow color. A small number of stibiotantalite crystals have been cut for collectors.

The properties of stibiotantalite vary, according to the variable ratio of tantalum to niobium. The material resembles sphalerite, but its optic character, specific gravity and strong doubling will serve to separate it from sphalerite.

The main source of gemmy material is Mesa Grande in San Diego County, California, where it occurs in pockets in gem tourmaline deposits. It is also found in Madagascar.

Since stibiotantalite is somewhat sensitive to heat, it must be dopped very carefully. It laps easily and smoothly and polishes well on tin with Linde A. However, differences may occur in the grinding speed, since the direction parallel to the broad face of
the crystal is softer than other directions. Also, the cut should be shallow because of the dark color and high R.I. Crown angles of 32° and pavilion angles of 40° are recommended.