Moonstone is on of three birthstones for June (the other June birthstones are pearl and alexandrite). It is a feldspar that is prized for its transparency and adularescence. Adularescence is caused by the diffraction of light as it hits thin, alternating layers of crystal within the gem. This produces the appearance of a floating, cloud-like, blue to white light inside the gem which is typically found in cabachon cuts.
Moonstone has a hardness of 6-6.5 and is somewhat fragile. However moonstone is plentiful and reasonably priced.