Moonstone is a variety of feldspar with a wide variety of colourations. It's formed of orthoclase feldspar and albite feldspar which have mixed and then cooled into thin, alternating layers. These layers 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. Sometimes, due to internal flaws, rough shape, inclusions, or pigment, the adularescence of a moonstone maybe so subtle as to simply look slightly shinier than usual.
Adularescence is not the same thing as "flash," which is the phenomenon of coloured light appearing on or within a stone. Colourless moonstone may exhibit blue flash. When it is able to exhibit multiple colours of flash (usually greens and yellows), it is called "rainbow moonstone." Rainbow moonstone is able to exhibit these colours due to inclusion of labradorite. For this reason, the stone has also been called "white labradorite."
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 has been thought to give protection to sailors.
It is thought that moonstone can activate intuition and help calm emotional turmoil.
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, sailing, space, water