Rainbow moonstone

Rainbow moonstone is the trade name for pale, transparent to translucent labradorite. Over a colourless or white body it shows a floating sheen that is blue and often many-coloured as the stone is turned. According to GIA the name is misleading: true moonstone is a variety of orthoclase feldspar, whereas rainbow moonstone is labradorite, a member of the plagioclase group. Even so, the trade has accepted it as a gem in its own right. Its main sources are southern India and Madagascar.

Jewellery: Its main use is in jewellery. The sheen shows best in dome-shaped (cabochon) stones, so it is mostly cut that way, and it is also made into beads. The very transparent Madagascan material is also faceted, in shapes such as rounds, hearts and hexagons. An alternative to moonstone: According to GIA the trade has accepted the stone as a gem in its own right, and today some buyers prefer it to traditional moonstone.

It has a hardness of 6–6.5 on the Mohs scale. It can scratch glass; a steel knife scratches it only with difficulty. Its specific gravity is 2.68–2.72 g/cm³.

Its best-known localities: Andilamena, Alaotra-Mangoro Region, Madagascar; Southern India; Sri Lanka; Tanzania.

The body is colourless, white or milky white; the sheen is seen only from certain angles and, besides blue, can show violet, green, yellow or orange. The sheen of true moonstone is mostly white or blue, while rainbow moonstone shows several colours together. That difference is the first clue for telling the two apart. Its refractive index and specific gravity are higher than those of orthoclase moonstone; a gem laboratory makes the final call with these measurements or with Raman spectroscopy. GIA has examined assembled stones made to imitate rainbow moonstone: a colourless rock crystal (quartz) base, an iridescent coating on top of it, a fibrous piece of gypsum above that and colourless plastic over the whole thing. Gas bubbles in the plastic layer and a blue glow under ultraviolet light give the imitation away.

On the “street to diamond” value ladder it sits on step 5 of nine: Ornamental stone. What matters most is how transparent the body is and how strong the sheen is: a bright sheen spreading across the whole face of a colourless, clean stone, showing orange and other colours as well as blue, is the most sought after. Cloudiness, black inclusions and cracks lower the value, and the stone must be cut so that the sheen shows from above. Only a small share of the mined material is good enough for fine gems and most stones are small, so large, transparent stones with a lively sheen are rare.

Today crystal-therapy websites link rainbow moonstone to the “third eye chakra” and claim that it strengthens intuition and spiritual awareness. The same sources claim that the stone soothes stress, evens out emotional ups and downs and is a protective talisman for travellers and new ventures. The colours of rainbow moonstone are a physical effect of light interfering in extremely thin feldspar layers inside labradorite; there is no scientific evidence that it strengthens intuition, calms or protects. The lunar and “feminine energy” meanings attached to it were carried over from moonstone purely because of its trade name, although it is not a moonstone at all. This section reports beliefs; it is not health information. There is no scientific evidence for these claims and no stone replaces medical treatment.