The Moonstone is a variety of Feldspar and, as its name suggests, is considered to be in close affinity with the Moon, because its pale lustrous blue colour resembles moonlight, which is believed by the natives of India to give the stone its colour. Indians believe that the best variety of the blue Moonstones are washed up by the tides when the Sun and Moon are in very harmonious relation, at intervals of twenty-one years (three periods of the Moon whose number is seven), and from this has arisen the saying of "Once in a blue moon" - to express a lengthy period.
It was known to the Ancients as Selenite, and Camillus Leonardus says it is powerful in recontiling lovers, and helpful to consumptives when the Moon is increasing in light, but when the Moon is waning, its stone will only enable its wearer to foretell future happenings; he also tells us that if the Moonstone be held in the mouth it will decide which affair should be taken in hand and which left alone; if to be undertaken, the matter is firmly fixed on the mind; if not, it passes out and is soon forgotten.
Pliny asserted that this stone contained an image of the Moon which waxed or waned according to the state of that luminary.
The virtues attributed to this stone were to protect from harm and danger in travelling by sea and land; to give mental inspiration, and to bring success and good fortune in love; also to preserve its wearer against dropsy and other watery diseases to which the Cancer type is liable.