The feldspar group can be divided into two isomorphic series: the alkali-feldspars and the plagioclase feldspars. Feldspars share a similar crystalline structure while their chemical compositions vary slightly among the different varieties. Feldspars are the most abundant minerals in the Earth’s crust.
Most gem quality feldspars are prized for their beautiful optical phenomena. Varieties include moonstone, labradorite, sunstone, Oregon sunstone, amazonite, orthoclase, and andesine. Their respective appearances and special features are described below.
Where can you find the feldspars you need ?
Pierres de Charme carries a vast selection of fine quality feldspars especially moonstone. Our collection of well-cut stones includes a variety of shapes and sizes.
- Moonstone is undoubtedly the most popular feldspar. Fine quality moonstones show a bluish to silvery reflection effect called adularescence. This effect is caused when light is reflected from the intergrown layers of different composition within the stone. Moonstone can be colourless, white, pink, yellow, green, brown, or grey. The reflection effect is most impressive when contrasted against a colourless body colour. Moonstone is usually fashioned as cabochons however, rare transparent material can be facetted. Moonstone can also show a cat’s eye effect. Most moonstones comes from India and Sri Lanka but there are also deposits in Myanmar, Brazil, Madagascar, Tanzania, and the United States. Moonstone is one of the birthstones for the month of June. According to Hindu mythology, moonstone is made from solid moonbeams.
- Labradorite can be dark blue to grey or yellowish to almost colourless with an iridescence effect displaying blue, yellow, and sometimes red reflections. Labradorite can be opaque to transparent and is generally fashioned as cabochons. Transparent labradorite with multi coloured reflection effects is known commercially as ‘rainbow moonstone’. Opaque material can be fashioned into tiles or decorative carvings. Labradorite may also have dark mineral inclusions. Major producing countries include Australia, Canada, Finland, Madagascar, Mexico, Russia, and the United States.
- Oregon sunstone
- Oregon sunstone can show many different colours such as pale pink, pale yellow, orange, red, and green. It can be transparent to opaque and may display a spangled effect due to small copper inclusions. Oregon sunstone and sunstone (oligoclase) have different chemical compositions and owe their colours and optical effect to different inclusions.
- Sunstone (Oligoclase)
- Sunstone, also known as oligoclase feldspar, can be colourless, orange, or brown with a spangled effect called aventurescence. This shimmery reflection effect is caused by many small hematite or goethite inclusions. Sunstone can be transparent to opaque. Sunstone deposits are found in Canada, India, Norway, Russia, and the United States.
- Amazonite, also known as microcline, is a translucent to opaque pale green to bluish-green gem with white veins and a silvery reflection effect. It can be fashioned as beads, cabochons, or small decorative sculptures. Amazonite is a porous material that can be impregnated with wax or oil to improve its appearance. Amazonite deposits are found in Canada, Australia, Madagascar, Russia, Mozambique, South Africa, Tanzania, Russia, and India.
- Orthoclase, although not commonly seen in jewellery, is popular with gem enthusiasts and collectors. It is a transparent colourless to yellow gem that is often facetted. Deposits are found in Madagascar, Myanmar, and Sri Lanka.
- Andesine is a pale yellow to red variety of feldspar mostly found in Tibet. The bright red colours can be the result of copper diffusion treatment.
Potassium, sodium, or calcium aluminium silicate
Monoclinic or Triclinic
Good in 2 directions
Between 1.51 and 1.57 (varies depending on the variety)
0.004 to 0.009
Between 2.56 and 2.75 (varies depending on the variety)
Value and Quality Criteria
Size, clarity, colour saturation, and especially the quality of the optical phenomena will have an impact on the value of feldspar varieties. Different varieties can have significantly different values.
Moonstone History and Lore. (n.d.). Retrieved August 06, 2016, from http://www.gia.edu/moonstone-history-lore
Fel-3. In Cours de base en gemmologie Gem-A. London, 2010
Fel-4. In Cours de base en gemmologie Gem-A. London, 2010
Fel-5. In Cours de base en gemmologie Gem-A. London, 2010
Fel-1. In Cours de base en gemmologie Gem-A. London, 2010