Stishovite
Stishovite is an extremely hard, dense tetragonal form of silicon dioxide. It is very rare on the Earth's surface; however, it may be a predominant form of silicon dioxide in the Earth, especially in the lower mantle. Wikipedia
Optical properties: Uniaxial (+)
Crystal system: Tetragonal
Density: 4.287 g/cc
Crystal class: Ditetragonal dipyramidal (4/mmm); H–M symbol: (4/m 2/m 2>/m)
IMA symbol: Sti
Melting point: (decomposes)