Petalite, also known as castorite,'s a lithium aluminium tectosilicate mineral LiAlSi4O10, crystallizing in the monoclinic system. Petalite's a member of the feldspathoid group. It occurs as colorless, grey, yellow, yellow grey, to white tabular crystals and columnar masses. Occurs in lithium-bearing pegmatites with spodumene, lepidolite, and tourmaline. Petalite's an important ore of lithium. The colorless varieties are often used as gemstones. Discovered in 1800, type locality: Utö Islan… (More on Petalite)