Moonstone, with its alien-like shimmer, was theorised by the ancient Romans as being formed from frozen moonlight! This unusual gemstone shines with a mysterious lunar light but it is actually an earthly mineral called feldspar. The shimmer, which is called adularescence, is caused by the inter-growth of two different types of feldspar, with different refractive indexes.
The alternating layers have slightly different optical properties and light passing through them is reflected and refracted at their interfaces. The reflected light waves interfere to produce the adularescence colours.
Moonstones come in a variety of colours. The gems colour ranges from colourless to grey, brown, green, pink or yellow. Moonstone also ranges from transparent to translucent. The most desirable moonstone has a blue sheen, perfect clarity and colourless.
Sometimes moonstone will have an eye as well as sheen. Another related feldspar variety is known as rainbow moonstone. In this variety of labradorite feldspar, the sheen is a variety of rainbow hues, from pink to yellow, to peach, purple, and blue. Sometimes one gem will show all these colours.
Fine moonstone is quite rare and becoming rarer. It is mined in Sri Lanka and Southern India. The rainbow variety can be found in India and Madagascar.
Moonstones are usually cut in a smooth-domed oval cabochon shape to maximize the effect. Sometimes they are carved to show a man-in-the-moon face.
Moonstone has a hardness of 6 to 6.5. It should not be stored in contact with your other gemstones to prevent scratching. Clean with mild dish soap: use a toothbrush to scrub behind the stone where dust can collect.