Titanite or sphene is a calcium titanium nesosilicate mineral, CaTiSiO5. Trace impurities of iron and aluminium are invariably present. Also commonly present are rare earth metals including cerium and yttrium.
The use of sphene as this mineral's name has been discredited by the International Mineralogical Association Commission on New Minerals and Mineral Names (CNMMN); titanite is the approved name (see CNMMN report, p. 134). However, sphene persists as the informal name for titanite gemstones.
Titanite, which is named for its titanium content, occurs as translucent to transparent, reddish brown, gray, yellow, green, or red monoclinic crystals. These crystals are typically sphenoid (from the Greek sphenos, meaning wedge) in habit and are often twinned. Possessing a subadamantine tending to slightly resinous lustre, titanite has a hardness of 5.5 and a weak cleavage. Its specific gravity varies between 3.52 and 3.54. Titanite's refractive index is 1.885-1.990 to 1.915-2.050 with a strong birefringence of 0.105 to 0.135 (biaxial positive). Titanite is a source of TiO2 used in pigments and is admired as a gemstone when suitably free of flaws.
Titanite occurs in metamorphic rocks such as gneiss and schists and also in granular limestones and granite. Transparent specimens are noted for their strong trichroism, the three colours presented being dependent on body colour. Owing to the quenching effect of iron, sphene exhibits no fluorescence under UV. Some titanite has been found to be metamict.
As a gemstone, sphene is usually some shade of chartreuse and is highly prized for its exceptional dispersive power (0.051, B to G interval) which exceeds that of diamond. Unfortunately, brittleness and low hardness precludes appropriate use as a ring stone: sphene is more suited to pendants or brooches, or left unset as a collector's stone.