Titanite, or sphene (from Ancient Greek σφηνώ (sphēnṓ) 'wedge'), is a calcium titanium nesosilicate mineral.  Trace impurities of iron and aluminum are typically present. Also commonly present are rare earth metals including cerium and yttrium; calcium may be partly replaced by thorium.  This specimen is from Morocco. 

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 in habit and are often twinned. Possessing a sub adamantine tending to slightly resinous luster, titanite has a hardness of 5.5 and a weak cleavage.

Titanite is a source of titanium dioxide, TiO2, used in pigments.

As a gemstone, titanite is usually some shade of chartreuse, but can be brown or black. Hue depends on iron (Fe) content, with low Fe content causing green and yellow colors, and high Fe content causing brown or black hues. Zoning is typical in titanite. It is prized for its exceptional dispersive power (0.051, B to G interval) which exceeds that of diamond.  Jewelry use of titanite is limited, both because the stone is uncommon in gem quality and is relatively soft.