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Moonstone – the lesser known June birthstone

Moonstone is the second in our trio of articles examining the June birthstones. The name is derived from the ‘moon like’ appearance of the light refracted from the micro-structure of feldspar layers on the stone’s surface, otherwise known as lamellae. The use of moonstone in jewellery dates back to the ancient civilisations. In Roman mythology moonstone was said to have been created by crystallised Moon rays themselves and the ancient Greeks, along with many others in antiquity, connected moonstone with their lunar deities. Moonstone experienced a revival of interest during the Art Nouveau period, circa 1890-1910. Most notably French jeweller René Lalique produced many fine collections containing moonstone. The most abundant variety of moonstone is the mineral adularia, which derives its names from the site of its early production in the vicinity of Mount Adular, near the town of Saint Gotthard, Switzerland. Moonstone is in fact feldspar with a pearlescent schiller, or bronzy, iridescent lustre and it is sometimes known by the alternative name hecatolite. It is actually comprised of two feldspar species, orthoclase and albite, which are intertwined in the gemstone to form alternate layers that refract and scatter the light in multiple directions, in a process known as adularescence. Moonstone deposits can be found in Armenia (predominantly in Lake Sevan), Australia, the Austrian Alps, Mexico, Madagascar, Myanmar (Burma), Norway, Poland, Sri Lanka, Finland, New Mexico, Viginia, USA, Mexico, Tanzania and India.

(An example of rare black moonstone)

Moonstone has been adopted by the State of Florida as its official gemstone, despite the fact it does not naturally occur there, to mark the successful lunar landing by Apollo 11 from the Kennedy Space Centre in July 1969. At 6-6.5 on the (1-10) Moh’s scale of hardness, moonstone is quite brittle so it can chip and crack quite easily. Despite this it is still a popular choice in jewellery designs, where cabochons and carved pieces are most popular, although some transparent stones are faceted. Due to its delicate nature, it is wise to set moonstone in as protective a mounting as possible. The most valuable examples generally display the highest degree of transparency and deep blue adularescence, whereas the more opaque examples with adularescent colouration other than blue are the least valuable. Some extremely rare cat’s eye and star (asterism) moonstone can command very high prices and are often difficult to source. Historically Burma was the origin of the finest colourless, transparent moonstone with a deep blue adularescence but today these mines are largely exhausted. Many specialist dealers and private collectors hold on to very fine moonstones in order to restrict supply and hence inflate the price of these rare gems. Moonstone has been revered by royalty and wealthy collectors for millennia and the highest demand for it comes from Germany and Scandinavia, where it is held in higher regard than pearl or alexandrite as the June birthstone.

Most moonstone are ‘cabbed,’ or cut, with high dome like tops to accentuate their natural lustre and this process maximises the potential to reveal a cat’s eye stone. When asterism does occur, the resultant star usually contains four points. As with many coloured gemstones, moonstone normally have inclusions and a completely non-included example is likely to be fake. Akin to emerald, they are characterised by the presence of fissures (or cracks). These are often found in pairs, running along the vertical axis of the crystal, accompanied by shorter cracks occurring perpendicularly along their length. Due to their appearance under the gemmologist’s microscope these are known as “centipedes” and they are the key characteristic looked for in lab tests to prove moonstone. Moonstones often contain rectangular dark patches, caused by stress cracking or discoloured crystals. These sometime link with a fissure to create a crescent-shaped inclusion and in the case of Burma (Myanmar) moonstone they often contain oriented needle inclusions. Some, very rare, moonstones display an elaborate multicoloured adularescence (sometimes referred to as a schiller), often containing blue with green and or orange. This phenomenon is sometimes referred to as labradorescence by gemmologists or as ‘rainbow moonstone’ as a more general term in the trade. Moonstone from Sri Lanka normally has a low specific gravity (the ratio of its density), whereas examples from India is slightly higher on the scale.

(When set into jewellery, moonstone must be in a protective mounting)

Due to the increasing value of moonstone some types of synthetic spinel and milky chalcedony (a microcrystalline quartz) have been sold as substitutes although specific synthetic moonstones are still very unusual. Large examples of fine quality moonstone is extremely rare but India produces a large quantity of relatively inexpensive moonstone of strong colour, which is often well cut and aesthetically pleasing to the eye. Moonstone displaying the highly desirable blue sheen are the most valuable and are very rare in sizes of over 15 carats but stones with a silvery or white adularescence are commonly found in sizes of up to many hundreds of carats. The largest known example was over 450 carats and it was discovered by the first Japanese expedition to Mount Kilimanjaro, Tanzania (formerly Tanganyika) in 1918. As with many coloured gems, moonstone is fairly brittle, at 6 on the Moh’s scale, it has a tendency to cleave (or break cleanly along distinct planes) so it pays to have it set very carefully into any piece of jewellery but rings especially. It is slightly safer when set into brooches, necklaces, pendants and earrings, because these do not usually come into strong physical contact with other objects. The key to looking after a moonstone is to protect it from any hard impacts that may cause it to chip, crack or even cleave into multiple pieces. It is vitally important to ensure you never use any form of mechanical or ultrasonic cleaning devices on a moonstone, as these can irreparably damage the stone. It is advisable to simply use a soft cloth, and if necessary a liberal amount of warm, soapy water, to clean moonstone gently by hand.

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