Gems and Stones

Feldspar Groups

The feldspar (sometimes written as felspar) group of minerals is the most abundant of all minerals and is widely distributed in igneous, metamorphic and sedimentary rocks.  Several varieties of the various groups supply gem materials.  Since I’m not a mineralogist, I won’t discuss the non-gem varieties here.

The feldspars, which are all closely allied in form and habit, are aluminous silicates of potassium, sodium, calcium or barium, but are practically without magnesium or iron.  The feldspars are usually divided into four subgroups:  (1) orthoclase and microcline (potassium aluminum silicate or KAlSi3O8); (2) albite (sodium aluminum silicate or NaAlSi3O8); (3) anorthite (calcium aluminum silicate or CaAlSi3O8); and (4) celsian (barium aluminum silicate).

Orthoclase:

The monoclinic potash feldspar (KAlSi3O8) is known as orthoclase.  In its purest form, it is known as adularia, is transparent or colorless and is of no gem significance.  The most important feldspar gem is moonstone.  The attractive sheen of moonstone is due to a combination of orthoclase and albite arranged in layers.  If the layers are too thick, the sheen is white and the stone is not very attractive.  If the layers are not too thick, the light reflected from these layers gives a light billowy effect called “adularescence”.  Moonstone can be semi-transparent to opaque, with a colorless to white body color – although it also occurs in green, yellow to brown, and gray to nearly black bodycolors.  Some specimens show chatoyancy or four-rayed asterism along with the adularescence.  A yellow variety of orthoclase is rich in iron and comes from the pegmatites at Itrongay, Madagascar.

Orthoclase has a hardness of about 6 on the Mohs’s scale.  It also has two directions of easy cleavage with are nearly at right angles to one another.  If you’re planning to cut a moonstone, the plane of the base of the cabochon must lie parallel to the plane of the layers to obtain the best effect.

Microcline:

The only gem variety of microcline feldspar is the verdigris-green to blue –green material to which the name amazonite or amazon stone is applied.  Microcline has the same chemical composition as orthoclase (KAlSi3O8), but the mineral differs from orthoclase in that it forms triclinic crystals.  The inclination of the third axis is very small and thus the crystals of amazonite strongly resemble those of orthoclase.  Microcline also occurs in translucent to opaque well formed crystals or as massive material of grayish-white, flesh to brown-red or green color.  Only the blue-green or green-blue material is used in beads and cabochons.  The material is not suitable for carving due to the easy cleavages.  The easy cleavage can be a danger in fashioned material which may break easily along the lines of cleavage if carelessly handled.  Amazonite is slightly harder than orthoclase and may reach nearly 6.5 on the Mohs’s scale.  The green microcline feldspar owes it variety name amazonite to the Amazon River, although it is not known to come from the vicinity of the Amazon.

Perthite, which owes its name to the town of Perth, Ontario, Canada where it was first found, consists of an intergrowth of albite or oligoclase in orthoclase or microcline.  When suitably cut, this material makes a handsome ornamental stone and exhibits a rich golden labradorescence against a flash-red to reddish-brown or white background.

Plagioclase feldspars:

The soda-lime feldspars or plagioclase feldspars constitute an isomorphous series between albite (the sodium aluminum silicate) and anorthite (the calcium aluminum silicate) and have a triclinic crystal system.  Mineralogists have applied various names to minerals falling into certain positions in the series:  Albite is 90-100% albite with 0-10% anorthite; Oligoclase is 70-90% albite with 10-30% anorthite; Andesine is 50-70% albite with 30-50% anorthite; Labradorite is 30-50% albite with 50-70% anorthite; Bytownite is 10-30% albite with 70-90% anorthite; Anorthite is 0-10% albite with 90-100% anorthite.

The albite variety of feldspar also produces a moonstone (the albite moonstone), but the variety is not at all common.  One type is called peristerite, sometimes called “pigeon stone”.  Peristerite is an ornamental feldspar with a white, cream, fawn or brownish-pink body color showing a celestial-blue flash of iridescence.  Transparent feldspar, nearly colorless with a tinge of blue or yellow, has been cut from material found in Kenya.

The gem mineral of the oligoclase variety of feldspar is sunstone or sometimes known as aventurine feldspar.  This mineral owes its attractive to the inclusions of red and orange (and occasionally green) microscopically thin platy crystals of goethite or hematite (or both) in the near colorless oligoclase matrix.

Andesine is a massive green material and somewhat resembles jade.

Labradorite (named for the coast of Labrador where it is found in extensive masses) is noted for the play of color (schiller) seen only on the easier of the two cleavage surfaces or on polished surfaces nearly parallel to this plane.  The most beautiful colors which flash out from the gray-colored labradorite are blues and greens.  Yellows, gold, reds and purple are also seen in labradorite.  The optical effect which produces these brilliant colors on the otherwise gray material is mostly due to interference of light.  Some labradorite has hematite inclusions like sunstone and produces an aventurescent labradorite.  A colorless labradorite, if darkened by needle-like inclusions and with a pronounced blue flash and some degree of chatoyancy when cut in the correct directions, is called “black moonstone”.

A clear, near-colorless to yellow transparent labradorite (which does not show a play of color) has been found in the US.  Some labradorite from Finland has been called Spectrolite, although this is also a trade name for labradorite with strong color flashes. A transparent to semi-transparent colorless labradorite with multicolored labradorescence has the misnomer of “rainbow moonstone”.

Bytownite has been found in red, pale yellow and “andalusite” colors.  Anorthite has been cut for collectors.  Laurvikite or larvikite (named from Laurvik in southern Norway) is a material extensively used for building facades and has a pearl-gray iridescence.

Labradorite sunstone, ranging in color from pale yellow, pale pink, medium orange and deep red to some rare greens, was discovered in 1980 in central Oregon and is being mined commercially.  Studies indicate the presence of rare pleochroism and aventurescence in these transparent and translucent gems, with the aventurescence due to native copper.

Care and Cleaning:

A final note for care and cleaning of your feldspar stones – NEVER clean them in an ultrasonic cleaner or use a steamer.  Warm soapy water is safe for cleaning your feldspars.