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 then experienced another great wave of popularity in the 1960s. 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 belongs to the labradorite branch. Moonstone's floating light that appears to emanate from within 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 en cabochon and the height of the cabochon must be correct in order to maximize the effect. Rainbow moonstone's labradorescence is a play of colors on the surface of the stone caused by the diffraction of light in the layers of rock. 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 unremarkable, giving the casual observer little idea of what constitutes their charm; that mysterious shimmer of light. This shimmer is not 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 the desired effect will be properly displayed.
Moonstone is found in Sri Lanka, Myanmar, Madagascar, Brazil, Australia and India. The various colors only come from India and the other sources yield white moonstone. 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 state of Florida 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 "moon town" in Thai. But the founding fathers of this Thai town did not take the opportunity to declare the moonstone a symbol, 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.