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Feldspar Gemstone Information

Feldspar is derived from the German words for field (feld) and split (splaten). It is a group of minerals that occurs in various colours, some with alluring optical effects.

Although most varieties are opaque and are cut en cabochon, labradorite and orthoclase varieties also occur as transparent stones and are faceted.

Oregon Sunstone, a transparent variety of labradorite, is the state gemstone of Oregon in the United States of America. The colour of Oregon Sunstone ranges from yellow and orange, to a rich deep red.

Another variety of feldspar known simply as 'Sunstone' is an aventurine feldspar (oligoclase). This variety is an opaque orange to brown stone with an attractive spangled or glittering effect.

Individual feldspar varieties are found in many areas of the world. Major sources for the feldspar group are Madagascar and the USA.

Physical Properties

Stone type:

Feldspars are a group of minerals that occur in two main subgroups. The varieties in each subgroup are as follows:

Alkali feldspars: Amazonite, Moonstone and Orthoclase varieties.

Plagioclase feldspars: Labradorite and Sunstone varieties.

Amazonite is a green to blue-green coloured stone similar to jade, it takes it's name from the Amazon river, although the Amazon is not a source location. Specimens can be a richly coloured bright green.

Moonstone has a sheen and displays an opalescent light effect that seems to move over the surface of the stone. This is called adularescence and resembles moonlight in a bluish-white colour. Some moonstones also show chatoyancy when cut en-cabochon.

Orthoclase is derived from Greek meaning "break straight" and refers to the gemstones cleavage, being perfect in one direction and good in another. The cleavages directions are 90 degrees apart.

Orthoclase is a pale yellow or champagne coloured stone and is normally transparent.

Labradorite named after Labrador in Canada, has a play of colour effect known as schiller that shows metallic colours of blue, green or golden yellow. The effect is also known as labradorescence. Rainbow labradorite is a highly translucent variety that can shows the full colour spectrum, when viewing the stone from different directions.

Sunstone is an translucent to opaque variety of feldspar also named adventurine feldspar (oligoclase), as it resembles adventurine quartz.  The colour is orange to red-brown and has a glittering or sparkling effect in red and gold colours.

Crystal System:
Monoclinic: moonstone and orthorclase.
Triclinic: amazonite, labradorite and sunstone.
Chemical Composition:
Alkali feldspars: Potassium sodium aluminium silicate
Plagioclase feldspars: Sodium calcium aluminium silicate.
Colour:

Amazonite: Green to blue-green, may have white streaks.

Moonstone: Colourless, white, yellow, green, orange and shades of brown and grey. The optical effect shown by moonstone may give highly desirable blue colours.

Orthoclase: Pale yellow colours.

Labradorite: Colourless, yellow, blue-grey to black, with a play of colour effect.

Sunstone: Colourless, orange and red-brown, with a red, green or blue glitter effect.

Lustre:
Vitreous.
Pleochroism:

None in most varieties. May be weak in adventurine (sunstone).

Oregon sunstone (a labradorite variety) may show red to yellow colours and green.

Dispersion:
Low.
Hardness:
6 on the Mohs' scale of hardness.
Toughness:
Poor.
Cleavage:
Perfect in one direction and good perpendicular to this direction.
Density in gm/cc:
Alkali feldpar: 2.56 to 2.60
Plaglioclase feldspar: 2.60 to 2.75
Double refraction:
0.004 to 0.009
Refractive index:
Alkali feldpar: 1.52 to 1.54
Plagioclase feldspar: 1.53 to 1.57

Common Treatments

Amazonite may be waxed or oiled, the rest of the feldspar group is not normally treated.


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