Feldspar | Gemopedia

While the feldspar is one of the most abundant mineral families in the world, gem quality crystals are scarce, coveted and spectacularly beautiful, often possessing rare optical effects. Many feldspar gems only occur in isolated deposits and are far rarer than better known gems such as diamond, ruby or sapphire. Some varieties are commonly encountered and many are seen in granites. “Black Pearl” granite countertops are actually composed mostly of feldspar.
Colors
Some varieties are known to have certain color ranges, such as the blue of amazonite. Many are colorless or show iridescent colors. Moonstones can be colorless or milky white, gray, yellow, orange to reddish, and even green. Labradorite is commonly a brownish gray color to almost black, but can also have no body color with iridescent flashes of blue or, more rarely, rainbow-like colors. Labradorite can also be a buttery yellow color. Sunstone is known for its metallic, iridescent look that is created by numerous tiny metallic inclusions of iron or copper. Oregon sunstones and andesine-labradorite can be green or red, sometimes a combination of both. Orthoclase is commonly whitish, but can occur in pink, yellow, blue or green.
Care
At 6-6.5, hardness falls in the mid-range for gemstones. Beads and cabochon-cut gems wear well, while faceted versions need more gentle use to avoid abrasion.
Shapes
Most feldspars are cut as cabochons to show their optical effects such as adularescence and iridescence or cat’s eye effects. Sunstones often show best in cabs, but highly translucent varieties such as some labradorites and andesines are beautiful when faceted.
Value
Rarely do feldspars attain the high per carat prices like some of the better known gemstones, but unusual colors, clarity and large sizes will command premiums based on their rarity. Purely colorless and clean gems are rare and highly prized as well.
Origin
The name feldspar comes from the German feldt spat, meaning “field stone,” because when feldspar weathers, it releases plant nutrients, such as potassium, which enrich soil.
Learn More
There are a wide variety of feldspar gem types and some are confusingly similar in appearance and composition. With ingredients such as sodium, aluminum, calcium, and potassium, their formulas can sounds more like a kitchen shopping list – or what is in your multi-vitamin. The various trade names can also confuse, as the same name is often applied to gemstones that are somewhat different in appearance, origin and composition. Amazonite, andesine, labradorite, larvikite, moonstone, orthoclase, and sunstone are all members of the feldspar family and are regularly used as gem materials, but there are more members that are less well-known such as albite, oligoclase, bytownite, microcline, sanidine, and anorthite. From a mineralogical perspective, the feldspars form an isomorphous series, meaning there is a range of feldspars of slightly varying compositions that are all similar in structure and chemistry. All feldspars fall into either the plagioclase or potassium category, depending on if they are potassium, calcium or sodium-rich in their formula. Their optical effects are another way to categorize feldspar, such as adularescence, labradorescence, aventurescence and iridescence. These effects are quite similar, but vary in use depending on which feldspar is being described. Adularescence applies to moonstones, also known as adularia, a type of orthoclase feldspar. Labradorescence applies to the iridescent spectral color flashes seen in labradorite and rainbow moonstone. Because of this effect, labradorite is occasionally called spectrolite. And while rainbow moonstone is actually labradorite (plagioclase) rather than moonstone (orthoclase), the name has stuck from years of traditional use. Aventurescence is the effect of the reflection off of many tiny plate-like metallic inclusions such as seen in sunstones. Sunstones can be from either feldspar group, as long as they feature aventurescence. Oregon sunstone does not always have such inclusions, but will instead show beautiful reddish or green body color.
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