Sphene (sometimes called titanite) is a mineral that comes in green, yellow, white, brown or black wedge-shaped crystals (sphene means wedge in Greek). Sphene is used only rarely as a gem (due to its relative softness). It's chemical formula is CaTiSiO5, Calcium Titanium Silicate. Sphene has a hardness of 5-5.5, a specific gravity of 3.3 - 3.6, and a white streak.