disphenoid

from The Century Dictionary.

• noun A crystalline form of the sphenoid group of the tetragonal system, bound by eight similar isosceles triangles in symmetrical pairs; a tetragonal scalenohedron. See sphenoid, 1.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective mineralogy Describing a wedge-shaped crystal form of the tetragonal or orthorhombic system.
• adjective mineralogy Describing a crystal form bounded by eight scalene triangles arranged in pairs, constituting a tetragonal scalenohedron.
• noun geometry A polyhedron whose four faces are identical isosceles or scalene triangles.