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Moonstone is the traditional and mystical birthstone for the month of June, as well as the ayurvedic birthstone for the month of September.
Moonstone is an opalescent variety of feldspars including orthoclase, anorthoclase, sanidine, albite, and oligoclase. The iridescence, called "schiller", results from the minute interlayering of different feldspars that develop by internal chemical separation during the process of crystalization. Very thin layers yield a blue sheen, much prized in jewelry, while thicker layers give a white sheen.
Among the most important silica groups is the feldspars. A group of aluminosilicate minerals that contain calcium, sodium, or potassium, they are the most common minerals in Earth's crust. Feldspars are the major component in most igneous rocks and are found not only on Earth but also on the moon and in meteorites.
During the earliest traditions in India, the gem was said to have been embedded in the forehead of Ganesh, the four-handed god of the moon, since the beginning of time. According to Hindu mythology, moonstone is made of solidified moonbeams. At one time it was believed that if you held one in your mouth during a full moon, you would see your future. In India, moonstone is still regarded as a sacred stone and is widely believed to bring good fortune. Merchants couldn’t display this gem for sale unless it rested on a yellow cloth. Yellow was the most sacred color. Moonstone also has a historical connection with travel. Once known as the "Traveler's Stone", it was used for protection against the perils of travel--especially by night.
Many other cultures also associate this gem with moonlight, and it’s easy to see why. Both the Romans and the Greeks associated moonstone with their lunar deities. They considered the gemstones as possessing those properties traditionally associated with the moon; romance, femininity, intuition, dreams, and emotions. Its internal structure scatters the light that strikes it, creating a phenomenon known as adularescence. The visual effect is reminiscent of the full moon shining through a veil of thin, high clouds.
George Frederick Kunz, in his famous book The Curious Lore of Precious Stones, tells the story of a famous moonstone that allegedly displayed a white point that changed shape and size in accordance with the waxing and waning of the moon. This story was most likely based on the Roman natural historian, Pliny, who coined the name of this gemstone when he wrote that moonstone’s shimmery appearance shifted with the phases of the moon—a belief that held until well after the sixteenth century.
Adularescent moonstone was once called “adularia.” The name originated with a city in Switzerland, Mt. Adular (now St. Gotthard), which was one of the first sources of fine-quality moonstone. Currently, the most exploited sources of moonstone are in Myanmar’s Mandalay region, Sri Lanka, India in Jharkhand and Tamil Nadu, and Austria in specific locations around the Alps there. Other locations include Brazil, China’s Xinjiang region, Lapland in Finland, Norway, Chihuahua in Mexico, Nanto City Japan, and Australia.