Crystal system Monoclinic
Transparency Transparent
Luster Vitreous to pearly
Fracture Sub conchoidal
Cleavage Perfect basal
Specific Gravity 2.40
Hardness 6 to 6.5
Optical Character Biaxial + ; Double Refractive
Refractive index 1.502-1.518
Birefringence 0.016
Fluorescence Non fluorescent under ultraviolet(lw & sw); yellowish orange under x-ray, phosphoresces
Chemical Formula LiAlSi4O10
Comments Not attacked by acids
Streak uncolored

Petalite (PET-al-ite) is a lithium aluminosilicate with the formula LiAlSi4O10. The name is derived from the Greek petnlon, meaning "leaf", in allusion to the foliated, cleavable masses in which it usually occurs. The alternate name is castorite.

Petalite is transparent and colorless when found in crystals, which is rare since this mineral is usually massive. The transparent crystals have occasionally been cut into faceted stones purely as collector's pieces, but they have appeal because of their glassy appearance. The colored, massive material, which is occasionally suitable for cabochons, ranges from greenish-white to gray to reddish.

Sources of petalite are Sweden; Karabib in South-West Africa; Massachusetts and Maine in the United States, as well as Australia and Brazil.

Petalite is brittle but tough; its one perfect cleavage is difficult to develop. It is not sensitive to heat and grinds slowly but smoothly with some directional differences. Polishing should be done on lucite with cerium oxide or on tin with Linde A. Crown angles of 42° and pavilion angles of 43° should be used in faceting.