Of all the phenomenal gems -- those displaying unique optical phenomena -- moonstone is the most abundant and affordable. The popularity of this unique gemstone has tended to wax and wane just like the moon. It was a great favorite of Victorian and Art Nouveau jewelers, and experienced another great wave of popularity in the 1960's. But in many cultures moonstone has always been important, and few gems are associated with so much romantic folklore.
Moonstone is the most famous member of the feldspar group. Technically, moonstone belongs to the orthoclase branch of the feldspar family and rainbow moonstone is in the labradorite branch. Moonstone's floating light that appears to come from below is known as adularescence. It is caused by structural anomalies within the stone which refract or scatter incoming light. The size or thickness of these anomalies determines the color of the floating light. To achieve this effect, moonstone is usually cut cabochon, and the height of the cabochon must be correct to maximize the effect. Conversely, rainbow moonstone's labradorescence is a play of colors on the surface of the stone caused by microscopically thin twinning. These phenomena are usually only seen when the the stone is in a certain position relative to the source of light and the observer.
In their uncut state moonstones are quite undistinguished and afford little idea of what it is that actually constitutes their charm: that mysterious shimmer of light. For that shimmer is not really shown to advantage until the art of the cutter has been brought to bear. Classical moonstones are always cut as cabochons, the most important thing being the correct height of the stone. The cutter must also align the axes of the crystal precisely into the zenith of the stone, for that is the only way in which he will bring about the desired light effect.
Moonstone is found in Sri Lanka, Myanmar, Madagascar, Brazil, Australia and India. The various colors only come from India and the other sources are white. In India, rainbow moonstone is mined in the southwest and blue is mined at Bihar in the center of the country.
In 1970, the year after American astronauts took off from Florida and landed on the moon, the Florida state legislature adopted the moonstone as the official state gemstone. Ironically, moonstone is not mined anywhere in Florida, nor is moonstone found on the moon. Our home of Chanthaburi in Thailand would seem like a better choice to adopt the moonstone as a symbol. Chanthaburi means "city of the moon" in Thai. But the city fathers seems uninterested in the moonstone as a symbol for the city, despite the fact that Chanthaburi is one of the largest trading centers for colored gems in the world. Perhaps it's because the city has generated its wealth from sapphire and ruby, not moonstone.