They are a type of feldspar, which is the most common mineral
on earth. so what makes them special?
First like most gemstones, they are crystalline. But the gems consist of plates of two different types of feldspar, arranged alternately. Light shining through the stone is refracted by these plates, to emerge as a striking glow, which shifts unexpectedly as the stone is moved.
The best moonstones are transparent blue. However high quality stones are rare; many are translucent, with different colours.
The name comes from the old belief that the glow looks like moonlight. In India they were believed to bring good dreams. Moonstones are the birthstone for June.
Sunstones show a similar band of light, but the cause is completely different.
I am not aware of any major dangers here. Synthetic moonstone does exist, but is rare.
The stones are soft, and can be scratched by dust etc, so do not wear them when doing manual work. They should not come in contact with any jewellery cleaner, but can be washed with lukewarm water.