Consumers like us often appreciate the rare gems and minerals, as they are hard to come by. If you do come across a unique piece, you may find difficulties getting your sight away from it. Rarity is one of the value deciding factors for gems and minerals. When rarity comes with beauty, it creates a priceless piece. But for common minerals that are less appreciated can also come with beauty, for instance, the feldspar.

Generally, feldspar is a silicate mineral that has a simple chemical composition of X(AlSi4)O8, aluminum, silica, oxygen, and an additional ion being the X. The diversity of feldspar is mainly resulted in that the ion may be one of seven different elements.

However, many of those elements are not common and the main elements are potassium or mixture of sodium and calcium. Those main elements can further make up two branches of feldspar: the potassium-rich alkali series, and the calcium-and-sodium-rich plagioclase series. Although there are sub-categories, their chemical compositions were never definitive as part of nature’s mystery. The amount of potassium or sodium and calcium may vary even if they are placed in a specific category.

Generally, feldspar is a silicate mineral that has a simple chemical composition of X(AlSi4)O8, aluminum, silica, oxygen, and an additional ion being the X. The diversity of feldspar is mainly resulted in that the ion may be one of seven different elements.

However, many of those elements are not common and the main elements are potassium or mixture of sodium and calcium. Those main elements can further make up two branches of feldspar: the potassium-rich alkali series, and the calcium-and-sodium-rich plagioclase series. Although there are sub-categories, their chemical compositions were never definitive as part of nature’s mystery. The amount of potassium or sodium and calcium may vary even if they are placed in a specific category.

A Feldspar Group chemical composition classification triangle.

Within those two branches that are slightly different in chemical composition, it may differ in crystal structure too. Most of the feldspar members are either in the monoclinic or triclinic crystal system. Although the feldspar group contains numerous members, its gem species can be identified into four types: Orthoclase, Labradorite, Oligoclase, and Microcline.

Despite all the differences, feldspar regardless of species have an average hardness of 6 – 6.5 and also have two perfect cleavage directions. Its cleavages intersect at almost 90-degree angle and may affect the durability of the gemstone. Thus, feldspar gems are typically used in pendants or earrings instead of rings or bracelets that are more exposed to impacts.

There are many gem varieties of feldspar. More commonly we know of are the ones that have special optical effects, also known as phenomena that stand out from the ordinary ones. The majority of the feldspar in the trade are phenomenal gemstones. Those include moonstone, labradorite, and sunstone of two different species.

For non-phenomenal feldspar, it only consists of amazonite, transparent yellow orthoclase, and labradorite. Transparent yellow feldspars are uncommon in the market, unlike amazonite. This microcline species can be an alternative for turquoise because of its blue-green appearance. On top of that, amazonite also consistently has gridlike white streaks as an additional characteristic.

Amazonite rough

Phenomena is a gemstone’s special optical effects resulted from how light interacts with it. Entering light can either be scattered by the crystal structure, reflected by inclusions, or absorption and emission of light spectrum. As for phenomenal feldspars, they are known for their adularescence, labradorescence, and aventurescence.

Adularescence can occur in several gemstones, most notably the moonstone. During the growth of the moonstone, two different feldspar species overlap one another in microlayers. Those microlayers are responsible for scattering entering light in many directions that create a billowy sheen just below the surface of the moonstone. Part of the orthoclase species, it is one familiar gem in the trade and the public.

History records stated fine moonstones were sourced in a town of the Swiss Alps. The town was formerly known as Mt. Adular, hence the moonstones mined there was dubbed “adularia” after the town. The best valued fine moonstones are ones with blue sheen. There is another variety of moonstone, known as rainbow moonstone of the labradorite species. It has a rather transparent to translucent body. It displays a wider range of colors as if it is a labradorite.

moonstone orthoclase feldspar
A blue adularescence moonstone. Photo: The Gem Museum

One the other hand, labradorescence may look quite similar to adularescence but it has a different mechanics to it. Labradorescence is made through the diffraction of light that reflects off the gem’s microscopic twinned structure.

Unlike moonstone, labradorites are highly prone to twinning, and those reflected lights are highly directional too. Different patches of sheen on the same stone may have to be viewed in another direction to see its effect. Labradoresence is named after labradorite and only it shows this phenomenon. Typically labradorite will show blue sheen, but also comes together with other usual colours such as yellow and green, with purple and red are rarer colours.

labradorite feldspar
Labradorite. Photo: The Gem Museum

Lastly, aventurescence refers to as a shimmering effect caused by reflection of light through numerous tiny platelets inclusions. This phenomenon is also known as the Schiller effect. Platelets within the stone usually have a common direction, so it creates one simultaneous flash on where the inclusions can be found. Common platelet inclusions are yellow copper, hematite, or goethite, and they sometimes determine the colour of the stone. Aventurescence can be widely seen in sunstones where it is most widely known for.

Sunstone with hematite inclusions. Photo: Silverhawk

Three feldspar species, oligoclase, labradorite, and orthoclase, have their sunstone variety. The sunstone labradorite is the most popular of the three in recent years. Sourced from Oregon, which was named Oregon Sunstone, often have highly reflective yellow to orange copper platelets. The coppers are typically the same size and minute enough so that it does not overpopulate the stone, and the exceptional brightness of light reflection can catch anyone’s attention. There may be many overlapping names across different feldspar species. It is important to understand the appearance of the stone so that we will not be confused and misled.

There are many reasons to own any feldspar. Even though they are soft and vulnerable to cleavages, their features unique to themselves are too good to be ignored. They can be cabochon or carved pendants to be worn around the neck, an ideal spot to catch light and showcase its phenomenon. Apart from being a jewellery, feldspar can also be decorative items, big or small, indoor or outdoor. Labradorite is a good choice for free-form free-standing pieces, as its rough can be large.

labradorite feldspar
Labradorite palmstone with multiple colours. Photo: Sagegoddess

Like any gemstone, feldspars have the unique beauty of their class. As a trade saying goes, no two gemstones are the same. Each piece holds its unique characteristics, be it physical properties, inclusions, or phenomena as their identity. Feldspar also can be an affordable universal gift for anyone to spark interest in gems and minerals. For loved ones, in particular, its sentiment value allows the receiver to treasure it dearly.

All in all, the feldspar group consists of many beautiful and unusual gemstones. Although it is dub as a common mineral, fine quality gemstones can still be as rare.

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