Please be aware that many sellers are offering 'Moonstone' items which are synthetic rather than real.

Moonstone, like Labradorite is Feldspar and resembles a lighter coloured Labradorite, sharing the iridescent quality of that stone.

It often has patches of dark stone and internal facets which catch the light, giving off a blue iridescence.

The synthetic moonstone appears slightly milky blue in colour and provides a rainbow effect, coloured  according to the light in which it is placed.
It is made mainly of glass and often contains bubbles revealing its true nature.

An honest trader will describe the item as OPALITE SYNTHETIC 'MOONSTONE' and may even describe the process of manufacture.

Others may try to get away with 'OPALITE MOONSTONE' using the trick of some Chinese sellers: a 'lapis' ring turns out not to be lapis 'material' just lapis 'colour'.  Hence the 'MOONSTONE' will be excused as indicating the colour or effect rather than the material.

As some sellers appear to have no idea that they are being misleading, I thought it relevant to warn buyers .  Google images clearly show the differences. ;)