Moonstone is a variety of feldspar with a wide variety of colourations, all of which have the opalescent quality that gives it its name. It's formed of orthoclase feldspar and albite feldspar which have mixed and then cooled into thin, alternating layers. These layers both scatter light similar to the way ripples in water will interfere with each other, causing the phenomenon known as adularescence, which itself is described as an effect similar to moonlight on water. This is also the phenomenon behind the chatoyant effect behind "cat's eye" gemstones.
The diffractive properties of the stone caused ancient peoples to believed moonstone to be solidified moonlight. Because of this, it has been popular in jewelry since ancient times, and it was associated with Selene and Luna as well as Hecate, from whom it gains the name "hecatolite." It was also a very popular gem in Art Nouveau jewelry and decor.
Because of its connection to the moon, it would be a good stone for Cancers.
Possible correspondence: Cancer, cats, duality, Greece, Hecate, illusion, Luna, moon, moonlight, night, Rome, Selene, space, water