Sphene

The fire of calcium titanium silicate (titanite) crystals can exceed that of much harder jewels. Facets appear to be doubled when viewed through the crystal. Three distinct colors can be seen depending on the angle the viewer looks at the crystal: nearly clear; greenish yellow and reddish yellow.

See also Matter, Gems, C and Copper Hydroxyls.

— Scotus - 30 Dec 2001