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Over the last two weeks we covered the gemstones in the “Alkali” Feldspar sub-group. This week I will explore the Gems in the Sodium/Calcium “Plagioclase” Feldspar sub-group. The minerals in this sub-group are similar and primarily differentiated by the percentage of Sodium vs. Calcium in their molecules. Most of the gemstones in this sub-group are known for their optical phenomena, and this is produced by internal light interference caused by the intertwining of two, or more, related feldspar minerals in their crystal lattice. Because of this, I will mention each of the minerals in this sub-group, and go in depth in describing the gemstones.

Below is a listing of the different minerals in the “Plagioclase” sub-group. They are listed in the order of their Sodium/Calcium percentages.

The Sodium Calcium “Plagioclase” Feldspar sub-group– (Sodium/Calcium
Aluminum Silicates).
Oligoclase (Rainbow Moonstone)
Andesine
Peristerite – (Labradorite)
Bytownite
Anorthite
Sunstone
Between the two sub-groups is Albite – (Sodium Aluminum Silicate).

The first mineral in this sub-group is Oligoclase. Oligoclase is second, only, to Albite in its’ high percentage of sodium vs. calcium (90-70% sodium to 10-30% calcium). As a gemstone it has three different forms:

1) Gem Oligoclase - A clear, colorless to yellow crystal that is usually cut into a faceted gemstone. Although these cut gems can be used in jewelry, they are primarily kept in gemstone collections.
Gem Oligoclase is found in the Ural Mtns, Russia

2) Rainbow Moonstone – aka, Oligoclase Moonstone. This transparent to translucent white gem with a blue shimmer floating across its’ surface is one of the most popular forms of Moonstone. The blue, moonlike shimmer is created by the gems’ scale like internal structure that causes light rays entering the stone to refract and scatter into a blue shimmer

A century ago, during the Art Nouveau period, the French jewelry artist, Rene Lalique and his contemporaries, would feature Rainbow Moonstone in their wonderful creations. Today, these pieces of classic jewelry are primarily found in museums and private collections.
The classic source is Sri Lanka, but it is also found in the USA; Brazil; Australia; Myanmar; Madagascar

3) Sunstone – aka, Aventurine Feldspar. This opaque to translucent to transparent, glitter filled, Oligoclase gem can have a reddish or golden shimmer floating across its’ surface. Small, plate like, inclusions of hematite or goethite produce the colorful “glitter” that can define Sunstone. Light interference, caused by these inclusions, produces the glitter colors – red and gold are common, green and blue are more rare. Sunstone is usually cabochon cut, but some of the most beautiful transparent stones are facet cut.

Sunstone is found in: Oregon, USA; Colorado, USA; India; Russia

Peristerite – (Labradorite – Spectrolite). Labradorite has a spectacular, metallic tinted, schiller effect known as Labradorescence, floating across the surface of this colorless to gray to brownish gemstone. The most common schiller colors are blues and greens, but golds, reds, purples, and a full spectrum of colors can be seen. Spectrolite is a most spectacular Labradorite from Finland.

Labradorescence is produced by the interference of light. This interference is caused by light bouncing off microscopic layers of different forms of Plagioclase Feldspar within the gems’ crystal lattice. Because labradorescence is produced by light bouncing off of layers within the stone, this gem has a “grain” and must be aligned before cutting, although, a cross-grain cut stone is, also, quite beautiful.

Labradorite is found in Labrador, Canada; Ukraine; Madagascar; Finland; USA; Mexico; Australia

Bytownite – This colorless to light pale yellow to champaign gem is usually faceted. Cut Bytownite is both set in jewelry and held in rare gemstone collections.

This gem is found around Bytown – (now Ottawa), Canada; Japan; USA, (New Mexico – transparent red).

This completes the Feldspar family of gems and minerals.

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