Leucite

Leucite, or White Garnet, named from the Greek leukos, "white," is a silicate of alumina and potash, of considerable interest among volcanic minerals. It occurs in very regular white or grey crystals, sometimes an inch' in diameter, having 24 trapezoid faces, the form being known as leucitoid. Optical characters show it to belong to the Pyramidal system; but when heated its angles undergo a slight change, and it becomes Cubic. Showers of crystals of leucite as big as pens are sometimes thrown out by Vesuvius; and they occur also in lavas known as leucitophyres, one of which near Rome has for centuries been used for millstones.