Clearly a beautiful gemstone and probably one of the most loved and cherished ones!
Rainbow Moonstone is the name given to a variety of labradorite that exhibits a blue or multicoloured adularescence on a light body colour. Rainbow moonstone is a member of the feldspar group, which makes up approximately 60% of the Earth's crust.
True moonstone is orthoclase (potassium feldspar), rather than labradorite (plagioclase feldspar). Though these two moonstones are related, they are technically not the same material. The reason for rainbow moonstone being referred to as "moonstone" is due to its adularescence, which according to some deems it worthy of the name. However, the adularescence of rainbow moonstone is caused by the same phenomenon as labradorite (reflection from twinning planes), where the true orthoclase moonstone gets its unusual adularescence from albite inclusions.
Besides all these great informations rainbow moonstones are one of my favourite gemstones. They are unique, no two are ever the same!