Feldspar
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 having alluring optical effects.
Although most varieties are opaque and are cut en cabochon, labradorite and orthoclase varieties also occur as transparent stones that are faceted.
Oregon Sunstone, a transparent variety of labradorite, is the state
gemstone of Oregon USA. 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). Sunstone 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.
Properties of the Gemstone and Additional Information
| Gemstone 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, that 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 it's bluish-white colour and effect. Some moonstones also show chatoyancy when cut en-cabochon. Orthoclase is derived from Greek meaning "break straight" and refers to the cleavages that is 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 a 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 and good in two directions |
| 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
Most stones are not normally treated, although Amazonite may be waxed or oiled.