Amazonite, Andesine, Moonstone, Oregon Sunstone, Labradorite, Oligoclase, Orthoclase and Sanidine* are rare, and often quite valuable, gems from the feldspar family of minerals.

Feldspar itself is extremely common, and there are dozens of types distributed worldwide. Although the larger family is commonplace, crystals in the feldspar family that are suitable for gems are scarce, quite valuable, and spectacularly beautiful. Many of these gems only occur in isolated deposits and are much more rare than the more widely known diamond, ruby or Tanzanite gemstones.

There are a wide variety of feldspar gem types, and some are confusingly similar in appearance and makeup to each other. Added to this, the trade names and common names often used for these stones can add to the confusion, with the same names often being applied to gemstones which are completely different in appearance, origin and mineral composition. Let's try to sort through the various types.

First, the technical side...

The Plagioclase group of feldspars encompasses most of the feldspar gem types. It is also used as a general classification for stones which do not seem to fit neatly into other sub-groups. Geologists distinguish among the plagioclase types based on the calcium and sodium content - beginning with Albite, which starts at pure sodium aluminosilicate. Then, as the sodium level drops and the calcium level increases, we go to Oligoclase, then to Andesine, then to Labradorites, then to Bytownite, and finally to Anorthite (a pure calcium aluminosilicate where there is no sodium). The specific Sodium/Calcium ratios which define each type are shown in the listing of gems which follows.

The alkali/potassium feldspars comprise a second sub-category. This grouping encompasses Microcline, Orthoclase and Sanidine. Unlike the Plagioclase group, the alkali feldspars share the same chemical makeup, and are distinguished by their crystal structure.

Now on to the real world...

For those who are not familiar with at least the more common of the feldspar gem varieties, here is a brief rundown of the types you may encounter in the marketplace. If you are looking at a stone and confused as to its identity, ask the seller the type and origin. Be aware, however, that a few of these gem types are closely related with only subtle differences. As a result, there is some confusion and debate as to classification, and even an expert might not be willing to give a firm identification of the type and source of a stone. On the other hand, some sellers are mine owners or have close relationship with mining companies, which allows them to be very confident as to the stone's identity. Rather than being overly concerned with the specific sub-type, most of us focus more on the stone's beauty and personal appeal - i.e., a well-cut, high-grade yellow Bytownite and a high-grade yellow Oregon Sunstone Labradorite can look alike, and can be difficult to distinguish without expensive test equipment, but are both beautiful and nearly equal in value.

Albite

Normally not used alone as a gemstone, this whitish mineral can be seen, along with other feldspars, in some gems (such as Moonstone and Perthite, see below). Albite is distinguished from other plagioclase feldspars by its ratio of sodium vs. calcium content (Sodium=90%-100%, Calcium=0%-10%).

Amazonite

Although this was first named based on descriptions from expeditions to the Amazon (as the name suggests), most Amazonite originates from mines in the United States and other areas. These transluscent to opaque stones can range from an olive green to a bluish green, with the blue-green variety usually more highly valued. Deep blue-green Amazonite from Virginia and Russia is available in very high quality and is eagerly sought after.

Amazonite: Amazonite cabochon on a slab of Perthite (a relative, see Perthite topic)

Amazonite is a microcline from the alkali/potassium feldspar (K-Spar) group. Its hardness ranges from 6.0-6.5 on the Mohs scale.

Andesine

Named after the Andes Mountains in South America, where it was first identified, Andesine is found world-wide. The gem variety is limited to shades of yellow (most often a light yellow to slightly brownish straw color). Since 2002-2003, high prices have been charged for Andesines that have been touted as deriving their colors from Copper. These include an intense red, green, a reddish orange and shades in between. They are of unknown origin (variously rumored from the Congo, Tibet, China, Mongolia, though no source has been confirmed by independent geologists). Note: since I first wrote this guide a few years ago, it has been confirmed that the highly colored red, orange and greens are not natural, but rather yellow labradorite from Mexico which has been highly treated by heat and copper diffusion processes - a scandal that is sure to grow.

Andesine (from top): Red-Green with silk, Yellow, Red-Green color shift, Salmon-Green bi-color, Red (in center)

Andesine is distinguished from other plagioclase feldspars by its ratio of sodium vs. calcium content (Sodium=50%-70%, Calcium=50%-30%). Andesine has a hardness of 6.0-6.5 on the Mohs scale.

Anorthite

Faceted Anorthite gems are usually quite small, and sporadically available. Anorthite is distinguished from other plagioclase feldspars by its ratio of sodium vs. calcium content (Sodium=0%-10%, Calcium=100%-90%).

Bytownite

This gem ranges from water-clear to light brown. Most of the current production originates in Mexico and New Mexico. It is also found near Ottawa in Canada, where it was originally discovered.

These stones are sometimes labeled as Sunstone, Yellow Labradorite or Champagne Labradorite. While not as frequently seen as some other members of the feldspar family, Bytownite is a reasonably priced transparent gem which can be very attractive in jewelry with the right cut.

Bytownite: Mexico

Bytownite is distinguished from other plagioclase feldspars by its ratio of sodium vs. calcium content (Sodium=10%-30%, Calcium=90%-70%).

Heliolite (Sunstone)

Heliolite is the Greek word for Sunstone (Helios=Sun, Lithos=Stone), and both Heliolite and Sunstone are often used as catch-all names for various feldspar, and even non-feldspar, minerals. In the feldspar group, it has been applied to varieties of Oligoclase, Orthoclase and Oregon Sunstone.

Hyalophane

Rarely found as clean crystals large enough for faceting, Hyalophane originates in the Balkans. It is occasionally offered as specimens for mineral collectors. Transluscent to opaque, the colors range from colorless to light yellow.

Labradorite

Most Labradorite which is encountered comes from large deposits in Canada, Finland, Russia, India and Madagascar. Large slabs of the gray variety have been occasionally used for spectacular wainscotting and countertops.

Labradorites (from top): Oregon Sunstone with Copper Schiller, Transparent Yellow (Mexico), Blue Flash (Madagascar)

As a gem, Labradorite is usually found cut into cabochons or set as tessarae in mosaic jewels. Opaque to slightly transluscent, the stone itself is usually gray. It is valued for a rolling flash blue, red, green, and yellow - sometimes metallic and sometimes almost opal-like. The Spectrolite from Finland can have especially colorful flashes. 

The term Labradorite also encompasses transparent varieties, such as Oregon Sunstone and a more common, but attractive, transparent yellow Sunstone/Labradorite which can be found in California, Canada, Mexico and several other locations.

Labradorite is distinguished from other plagioclase feldspars by its ratio of sodium vs. calcium content (Sodium=30%-50%, Calcium=70%-50%). Labradorite has a hardness of 6.0-6.5 on the Mohs scale.

Moonstone

Gems under this name are usually plagioclase feldspar, and Moonstones are the most well-known gems in the feldspar family. They share an irridescent shimmer (known as adularescence). This effect is caused by seams of Albite crystals lying inside the main body (usually Sanidine or Orthoclase).

Moonstone: India

The chief source of Moonstone is Sri Lanka, although Moonstone gems can be found in several areas worldwide. Rainbow and Blue Sheen varieties have also come on the market. The more valued tend to be cat's-eyes and those with unusual coloring and transparency.

The traditional moonstone is a shimmering white. Other colors include beige, yellow, green, blue sheen, grayish and pink varieties (more Moonstone info).

Oligoclase or India Sunstones

Traditionally from India and Sri Lanka, these are the most common Sunstone encountered in jewelry, and are what many people recall when the word Sunstone is mentioned. This type is sometimes referred to as aventurine feldspar in the jewelry trade.

Oligoclase (from top/left): Cat's Eye, Yellow, India Sunstones, Confetti (Tanzania)

Opaque to transluscent varieties are often cut into cabochons or beads. Their chief feature is the sparkling Schiller flaking over the brown to red-orange surface. The Schiller in Oligoclase stones is composed of goethite or iron (hematite or pyrite). The Schiller in Oligoclase Sunstones is much coarser than the nearly microscopic particles of copper Schiller found in the Oregon Sunstone variety of Labradorite (see below). There is a wide variation in India Sunstones, and these can range from quite dull and uninteresting, to very bright and attractive.

In addition, there are similar gems having hematite Schiller flaking or silk which come from Africa, Russia and other regions which also are labeled as Sunstone  - including some cat's-eye and tiger's-eye types, and a very colorful Confetti or Illusion type from Tanzania.

Oligoclase is distinguished from other plagioclase feldspars by its ratio of sodium vs. calcium content (Sodium=70%-90%, Calcium=30%-10%).

Oregon Sunstone

Oregon Sunstones are generally a transparent, facetable form of Labradorite which display colors coming from copper. However, gems coming from some Oregon Sunstone mines have exhibited slightly more sodium, and less calcium, putting those into the Andesine classification. Few, if any, gemstones come in such a variety of colors and types.

While yellows and pinks are most often seen, the saturated, transparent reds have traditionally commanded much higher prices. Scarcer still are green, teal blue, and aqua stones, with intense shades encountered even less often. Another rarity are the dichroic or trichroic gems (which change between 2 or 3 colors respectively as the stone is moved - an effect is also known as pleochroism). Color change gems have also been found, and they are also quite rare. Like Alexandrite, color change sunstones display different colors, depending on the type of light (sunlight, incadescent, etc.).

Oregon Sunstone (from top/left): Bi-Color, Dichroic, Blue-Green, Orange, Pink, Clear. Medium-Red in center.

Many Oregon Sunstones exhibit a metallic flash known as Schiller which is caused by microscopic flecks of copper aligned in layers. The Schiller can be copper orange, red, silver or pink. The Schiller sometimes forms a pattern which flashes only from certain orientations, but is otherwise invisible. The Schiller stones are extremely varied.

Hardness of Oregon Sunstone ranges from 6.5 to 7.2 on the Mohs scale, making it suitable for a wider variety of jewelry types, than other, softer, gems in the feldspar family. I have also written a separate Guide to Oregon Sunstones which contains further information.

Orthoclase

A colorless to champagne yellow stone, the current chief source for gem-quality Orthoclase is Madagascar, with other deposits distributed worldwide. Transparent gems are sometimes sold as Sunstone or Candlelight Labradorite and can be quite attractive. Opaque to transluscent cat's-eye cabochons are also seen, with silky threads.

Orthoclase: Brazil

Perthite

This gem is similar in color and opacity to the related Amazonite, but with prominent webbing or veining (for picture, see the Amazonite topic above). Perthite is sometimes seem labeled as the more well-known Amazonite. Colors range from blue-green to pink to yellowish. Prominent sources are found in the United States (Colorado and Virginia), Norway, Canada, Brazil, India, Madagascar and Zimbabwe.

In some varieties, the webbing is so fine that it gives a Moonstone or cat's-eye appearance.

Perthite is a combination of microcline from the alkali/potassium feldspar group and plagioclase. The more colorful strips are microcline, with plagioclase (usually Albite) forming the white or transparent veining. Hardness ranges from 6.0-6.5 on the Mohs scale.

Red Labradorite

This broad designation has been used as a catch-all term for red Andesine, (whether Oregon Sunstone or from another source). This usage tends to occur when the stone's origin is in question and the evaluator either does not have the equipment for, or does not wish to do the tests necessary for further narrowing down a gem's type. An Andesine-Labradorite type has also been touted, which, although an individual stone may exhibit characteristics of both, precise testing must show it actually to be either Labradorite or Andesine - not as both types - even if a mine or specimen contains some of each.

Other than the Oregon Sunstone variety of Andesine and Labradorite, stones from non-Oregon locations in colors other than yellow or gold have thus far proved to be highly treated to produce their colors.

Sanidine

Somewhat rare in gem quality, deposits exist in Germany and Idaho. The faceted examples I have seen have been a light yellow. Because production has been too low to ensure a steady supply for the jewelry trade, this is mainly a stone for collectors and custom pieces.

Sanidine: Madagascar

* note that in this guide I've capitalized the names of these gem types to make it easier for novices to read. Properly, they would be left uncapitalized (although some allow capitalization where named after a person or place).

Other subjects on which I've written recent Guides...
Other Gemstones
Coin Collecting

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