Moonstone is a variety of feldspar which is the most common mineral in the earth’s core. It is technically a sodium potassium aluminium silicate and is characterised by it’s adularescence (it’s shine as light moves through it).
The shimmer of colour as light moves through it is due to the different layers of feldspar that make up it’s internal structure. These layers have different refractive indexs and therefore they slow light at different levels which creates refractioned light through one which is then refracted again through the other layers.
It is commonly mined in Australia and Sri Lanka but there have been numerous deposits of moonstone found the world over. Furthermore, it is not a new gemstone, there are records that show it was around during the age of the Romans, where they believed it was created by frozen moonlight. Although not a particuarly rare stone it is nonetheless a beautiful one and when cut into cabochon shapes they become very popular jewelery pieces.
With a rating of 6.5 on the moh scale they are hard enough to withstand some wear but they can easily be chipped or scratched so care must be taken when wearing one to not expose it to too many extrememe conditions.