Moonstones are neither rare nor expensive, but they're a beautiful opalescent stone used widely in jewelry-making. One type of moonstone is a composite mineral, made up of orthoclase and albite, two types of feldspar, and other mineral cousins also called moonstones are composed of labradorite and larvikite. Moonstones are translucent and come in a wide variety of colors, including yellow, gray, green, pink and brown. The moonstone has been valued for centuries and in Germany and Scandinavia is preferred over pearl as the birthstone for June.
The true moonstone is a variety of orthoclase and albite feldspar or sodium aluminum silicate. It's sold as a cabochon, a highly polished, smooth round stone with no facets. These moonstones can be colorless, white, blue white or pale shades of peach and apricot. The rarest and most expensive stone is irridescent and colorless with a floating blue color called the Schiller phenomenon. Cat's eye moonstones and star moonstones are formed from concentrations of narrow bands of reflected light across the center of the cabochon.
The Schiller phenomenon appears as multi-colored, showing blue with green and/or orange. These stones are quite valuable and are known as rainbow moonstones...not to be confused with the blue rainbow moonstones (see below), which aren't true moonstones. The most valuable moonstones are those that are colorless, transparent and have a strong blue sheen. Historically, these stones came from Burma but that source has been mined out.
Blue Rainbow Moonstone
Labradorite is not mineraologically a true moonstone, although that is what some in the industry call it. Blue rainbow moonstones came onto the jewelry market 15 years ago after its discovery in southern India. The labradorite feldspar has a blue sheen called royal blue or a multi-colored sheen called rainbow.
Blue Norwegian Moonstone
The blue Norwegian moonstone, also called Norwegian moonstone and black moonstone, is composed of larvikite, a mineral from the area around Larvik, Norway. The lighter variety is called blue Norwegian moonstone. Larvikite has many trade names such bird's eye granite and Norwegian pearl granite but it's a feldspar; it's not granite.