Sphene

Sphene is the older name for titanite, a calcium titanium silicate, CaTiSiO5. Wikipedia says the International Mineralogical Association adopted titanite and discredited sphene in 1982, and that sphene comes from a Greek word for wedge. Cunningham's heading is Sphene, with Titanite as its folk name. He describes a greenish yellow stone rarely found in transparent crystals and seldom used in jewellery because it is soft, and gives it Mercury and Air. Cunningham uses it to improve the mind and the handling of information, for studying, theorising and debating, and for spiritual enlightenment during meditation and mystic rituals. Wikipedia lists its crystals as reddish brown, brown, grey, black, yellow, green, red or colourless, and as a common accessory mineral of igneous and metamorphic rocks. No other magical source read has an entry for it.