Rainbow Moonstone

July 22, 2020

Feldspar minerals makeup a whopping 60% of the earth’s crust and moonstone has always been the favourite child among the feldspar family. Adored for its delicate beauty, moonstone has a rich heritage of uses in jewellery over the centuries, more notably during the art nouveau movement, dating back to ancient civilizations including the Romans and Greeks. Believed to have been created from solidified moonbeams it was a stone held in high regard as a tangible connection to the magic of the moon, a talisman of protection for travellers and today is known as the ‘stone of new beginnings.’ In fact, in more recent times during the Victorian times, moonstone rings were more used as engagement rings than anything else!

Recognised by its whimsical blue to silvery sheen (adularescence) is it the ‘rainbow moonstone’ that has become most popular and available over the past few decades. But can we technically call it ‘moonstone’?

Moonstone is a variety of the orthoclase series of feldspars with mineral constituents of albite and orthoclase. These two minerals alternatively layer each other in microscopic sheets in a fine structure referred to as lamellar twinning. The twinned layers are planes of weakness within the crystal’s architecture and may ‘cleave’ or ‘split’ with light to moderate impact. That billowy blue sheen is a result of light interference from the thin layers of feldspar minerals, produced by an optical effect rather than by its chemical composition. As white light strikes the stone’s surface some passes through to layers beneath and diffracts, some wavelengths of light are absorbed and the remaining is reflected light producing a vivid blue adularscence. Presto!

Similarly, in labradorite (moonstone’s sibling) the feldspar minerals albite and anorthite create a lamellar structure and result in a multi coloured sheen (labradorescence). Labradorite has a light to dark-grey body colour and is capable of displaying a sheen with all the colours of the rainbow, most often shades of velvety blue and sea green. The deep blue/violet colours are most desirable in the jewellery market but the most impressive in my opinion are those that show a full spectrum.

Found mostly in Sri Lanka, India, Australia, Myanmar and USA, rainbow moonstone became more popular in the 1960’s but it wasn’t until the mid 1990’s it launched into the jewellery scene in commercial quantities as ‘rainbow moonstone’ this transparent-colourless to translucent-white gemstone, found in Bihar State, India, displays a blue through to multi-coloured sheen. This unique gem often displays colour zoning where parallel bands and angular lines of different colours appear; this would be evidence of an internal structure where layers of the alternating minerals vary in thickness and therefor absorb/reflect the light of different wavelengths.

Further testing of this mineral has in fact shown it to be part of the plagioclase series of feldspars and that it is the lack of ilmenite inclusions (responsible for labradorite’s grey body colour) that causes it to be so similar in appearance to ‘regular’ blue sheen moonstone. Although chemically rainbow moonstone may be classified as labradorite it is similar enough that the name ‘rainbow moonstone’ is an accepted name for this mineral within the industry.

Due to this remarkable and delicate structure of these feldspar minerals (combined with a hardness of just 6) the most significant issue that may arise is parting/splitting along the twin planes whilst fashioning the rough into cabochons and faceted stones. Bunny Bedi, owner and director at Made In Earth, has had the joy of buying, cutting and selling this mineral since its first appearance on the market.

“I’ve always had a love-hate relationship with rainbow moonstone. Although it is undeniably beautiful it’s very delicate and stressful to work with. We keep out cab in oil to regulate the temperature to avoid cracks forming in the stones and of course setting is another challenge all together.”

“The price of rainbow moonstone has quadrupled over the last seven years and the high grade stones with a clear body and vivid blue sheen are almost impossible to find in large quantities. Luckily for us our clients appreciate the multi-coloured sheen (reflective of its name) and love the milky moonshine-glow quality some stones have.”

Rainbow moonstone is one of the standouts for special occasions and definitely something you want to have in your collection!

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