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
It is not known when or where pendulum divination first developed. In the modern era, pendulums like this one are typically used by assigning an answer to a particular direction of swing or a type of movement, then asking it a question and awaiting it to take on the answering movement. Most typically this is done with a yes/no question, with "yes" being a lengthways movement, and "no" being a crossways movement, but some versions allow for multiple directions, while others may use circular movements. Some people prefer to use a pendulum mat, in which lines are drawn from the same connection point, and each assigned a possible meaning.
Pendulums are also used to measure time and to focus the mind in meditation.