"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."
-- Camillus Leonardus, Sixteenth-century Physician
Moonstone is a type of feldspar known as orthoclase, with a monoclinic crystal system. Recovered from mines in Sri-Lanka, Switzerland and Burma, the mineral has "cleavage planes" that produce a silvery bluish-white chatoyancy - an almost cat's eye quality that changes in relation to the light reflected from its surface.
A REFLECTION OF THE MOON
Pliny mentions stones called astrion, astriotes and ceraunia, and these were very probably what we now know as moonstone. He describes the first two as being transparent, like rock crystal, but with a bright white spot that appears to move as the stone is rotated and twisted in the fingers. This spot was believed by the ancients to be a reflection of the moon - Pliny compares astrion and astriotes with "a star shining brightly like the full moon" - and the bright spot was thought to wax and wane in harmony with lunar movement.
According to Pliny, astrion and astriotes were so-named because, when held up to the stars, the stones collected and reflected their glitter. Pliny: "The best kind came from Carmania and were called 'ceraunia' (thunder stones). They imprison a bright star, and although in itself it is like rock crystal, has a brilliant blue sheen." He also spoke of dull ceraunia stones which "if steeped in soda and vinegar for several days from such a star which fades away after several months."
Camillus Leonardus lists the stone as selenites. It contains, he says, the figure of the moon or a clouded star and claimed that samples from Persia increase or decrease in color in time with the phases of the moon. He contains:
"Being put in the mouth, which must first be washed with water, such affairs are thought of as ought or ought not to be taken in hand. If to be undertaken, they are so fixed in the mind that they cannot be forgotten but if not, they soon vanish out of the mind."
ONCE IN A BLUE MOON
On the Asian continent, the pale lustrous blue color of the moonstone is considered to resemble moonlight. However, the best of the blue moonstones are washed up by the tides when the Sun and the Moon are in a particular heavenly and harmonious relationship, which occurs every twenty one years (three 7-year cycles of the moon). This event gives rise to the saying that denotes a lengthy period of time - "Once in a Blue Moon".