the 'star stone'
Sri Lankan legend says moonstone was first discovered by a person of Tamil origin sometime during the country's colonialization by the Portuguese (1505-1658) but it was probably known long before the Portuguese arrived.
moonstone exhibiting asterism
Also found in India but in much less quantity, it's very possible that Indian moonstone found its way to ancient Greece, Rome, and Egypt.
Pliny the Elder (23-79 AD), Roman historian, naturalist and author of the world's first encyclopedia, referred to stones called astrions -- meaning "star-stones" (also called asteria, or astriotes) -- which are believed to have been moonstones. He described them as "a colorless stone...[from India] having within it the appearance of a star shining brightly like the full moon."
The bright white spot that appears to move as the stone is rotated was believed by the ancients to be a reflection of the moon that waxed and waned in harmony with lunar movement. According to Pliny, when moonstone was held up to the stars, the stones collected and reflected their glitter. (The term "star-stone" later referred to sapphires which were probably unknown at the time.)
diana, goddess of the moon
Legend has it that amulets of moonstone, associated with the goddesses Isis
(Egyptian), Diana (Roman) and Selene
(Greek), were hung in fruit trees to produce abundant crops.
Moonstone was also thought to protect against wandering of the mind, insanity and epilepsy; and was attributed with improving physical strength and reconciling lovers. If held in the mouth, a moonstone was even thought to help one make the right decision.
Italian Renaissance Prince Cesare Borgia's court physician, Camillus Leonardus, author of Speculum Lapidum
-- "Mirror of Stones" -- (1502), called the stones selenites
"[Selinite] contains the figure of the moon or a clouded star... It is very powerful stone in the reconciling of love, and during the whole time of the increase of the moon, it helps the pthisical (consumptive); but in the decrease it discovers surprising effects, for it enables a person to foretell future events."