Moonstone - Learning Geology

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Saturday, December 24, 2016

Moonstone

What is Moonstone?

Moonstone is the most well-known gemstone of the feldspar group. Named for its glowing colour sheen that resembles the moonlight, Moonstone can belong to several different members of the feldspar group, especially Orthoclase and Oligoclase. Moonstone displays a unique play of colour known as adularescence. This effect is in the form of a moving floating light or sheen. This phenomenon is caused by structural anomalies within the crystal formation.

History and Introduction

Moonstone is the most well-known gemstone variety of orthoclase feldspar, a potassium aluminium silicate. It is a transparent to opaque oligoclase, a variety of plagioclase albite and sheet mica. Moonstone is known to exhibit a distinct sheen under certain lighting conditions, and it is the sheen which renders moonstone one of the most remarkable gemstones available today. In fact, its name is owed to the almost magical, bluish white shimmer it exhibits, which closely resembles that of the moon. Gemologists refer to the shimmering optical phenomena as 'adularescence'. The optical effect of adularescence is a result of moonstone's unique structural pattern. Tiny inclusions of albite, a sodium aluminium silicate are intermixed with host rock layers of orthoclase, a potassium aluminium silicate. The alternating layers of different feldspars form a lamellar (scaly) structure which causes the interference of light as it enters the stone. Thin layers of alternating silicates tend to refract more attractive and colourful sheens, whereas thick layers of silicates produce less attractive, white to colourless sheens. As light enters the stone, it is refracted and scattered, producing an extremely unique and attractive play of both colour and light. With moonstone, the aura of light actually appears to glow from deep within the surface of the stone.

World-famous Moonstone

Moonstone was extremely popular in the times of "Art Nouveau", which took place more than 100 years ago. It was used to decorate a striking amount of pieces of gemstone jewellery created by the famous French Master-Goldsmith, René Lalique, as well as many of his contemporaries. His rare pieces are typically only found in museums or in well-guarded private collections.

Identifying Moonstone

Moonstone can be identified by the presence of adularescence. Other gems with a similar appearance do not have the phenomenal presence of adularescence which makes identification of moonstone fairly easy. Moonstone is a potassium aluminium silicate and can be easily identified by composition. Many similar materials, such as labradorite, are actually plagioclase feldspar, whereas moonstone is by composition a potassium feldspar. Testing for hardness is often one of the easiest methods for distinguishing moonstone from other materials. Other similar gems, such as opal, chalcedony or ammolite, are significantly harder or softer than moonstone. Top quality moonstone can show an incredible "three-dimensional" depth of colour, which no other gemstone can replicate, making moonstone almost unmistakable.

Moonstone Origin and Gemstone Sources

Moonstone deposits are often found as constituents in feldspar-rich granitic and syenitic pegmatites all over the world. The most important moonstone deposits are from Sri Lanka and India. Other notable sources include Australia, Brazil, Germany, India, Myanmar (Burma), Madagascar, Mexico, Norway, Switzerland, Tanzania, and the United States. Sri Lankan moonstone is most famous for its attractive blue coloured material, but blue moonstone is becoming increasingly rare. India is known for producing fine 'rainbow moonstone'. Switzerland's Adula Mountains possess the most historically interesting moonstone mines.

Moonstone: Varieties or Similar Gemstones

Moonstone belongs to the large group of feldspar minerals, the most abundant minerals on earth. The feldspars make up approximately 60% of the Earth's crust, which means there are many moonstone-related gems and minerals. Quartz gemstones make up the second most abundant gemstone group, second only to the feldspar family. Feldspars are typically classified into two main gemstone groups: Potassium feldspar and plagioclase feldspar. All varieties of moonstone are potassium feldspar. Other potassium feldspar gemstones include amazonite and orthoclase. There are also several gemstones which are often confused with moonstone based on appearance alone, such as 'rainbow moonstone'. Rainbow moonstone is actually not a true moonstone, but rather, it is a variety of labradorite plagioclase feldspar. This is why it is sometimes referred to as 'blue-sheen labradorite'. Most Popular Similar or Related Trade Names & Gemstones: Star moonstone, cat's eye moonstone, rainbow moonstone and sunstone are the most popular and well-known trade names used for similar or related gemstones.Lesser-Known Similar or Related Trade Names & Gemstones:Orthoclase, amazonite, labradorite, andesine, andesine-labradorite and oligoclase are the lesser-known trade names used for similar or related gemstones.

Properties of Moonstone

Chemical FormulaPlagioclase Feldspar: (Na,Ca)Al1-2Si3-2O8Orthoclase Feldspar: KAlSi3O8
ColourWhite, Colorless, Blue, Green, Yellow, Orange, Brown, Pink, Purple, Gray
Hardness6 - 6.5
Refractive Index1.518 - 1.526
SG2.56 - 2.62
TransparencyTransparent to translucent
Double Refraction-.0005
LusterPearly
Cleavage2,1 - basal ; 2,1 - prismatic ; 3,1 - pinacoidal
Mineral ClassOrthoclase, Oligoclase, and other members of the feldspar group
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