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Oligoclase
Oligoclase is tectosilicate (plagioclase feldspar), (Na,Ca)(Al,Si)₄O₈ (An₁₀ to ₃₀). In the hand, it rates 6 to 6.5 on the Mohs scale, and it runs white and gray. The sodium-rich plagioclase behind classic Indian sunstone and blue-flash 'peristerite' moonstone, blocky feldspar with twinning striations on cleavage faces.
The geology.
Oligoclase carries the formula (Na,Ca)(Al,Si)₄O₈ (An₁₀ to ₃₀). It belongs to the triclinic system. It is classified as tectosilicate (plagioclase feldspar).
It sits at 6 to 6.5 on the Mohs scale. The surface reads vitreous to pearly. Light behaviour is transparent to translucent. Specific gravity runs 2.64 to 2.66. The streak is white. It breaks with two cleavages near 90°.
The sodium-rich plagioclase behind classic Indian sunstone and blue-flash 'peristerite' moonstone, blocky feldspar with twinning striations on cleavage faces.
You will most often meet it as blocky crystals, cleavable masses, sunstone rough or moonstone-like cabs.
The origins.
Granites and metamorphics worldwide; sunstone oligoclase from India and Norway; peristerite from Ontario (Canada).
On value, the database notes: common; gem sunstone grades are mid-range.
Traditional associations.
In energy work traditions, oligoclase shines through its sunstone form's joy-and-vitality associations.
People buy on this language, so it belongs on the page. It belongs under the facts, though, not over them.
Spotting the real thing.
The sodium-rich plagioclase behind classic Indian sunstone and blue-flash 'peristerite' moonstone, blocky feldspar with twinning striations on cleavage faces.
Goldstone glass impersonates its sunstone form constantly, uniform glitter means glass.
| Looks like | How to tell them apart |
|---|---|
| Orthoclase | Potassium cousin, oligoclase shows fine parallel twin striations on cleavage that orthoclase lacks |
| Quartz | No cleavage flash, glassier |
| Albite | Continuous series, labs draw the line; field names follow the deposit |
At-home tests.
You can run every one of these at a kitchen table. Good light matters more than gear when you are looking at oligoclase.
| Test | What you should see |
|---|---|
| By eye | Twin striations on cleavage faces (plagioclase badge) |
| By eye | Right-angle cleavage flash |
| Scratch check | Scratches glass with effort |
Stop short of anything that leaves a mark. A gemologist can answer the rest without damaging the stone.
Care & handling.
At 6 to 6.5 on the Mohs scale it handles careful wear but scratches against quartz and dust. Store it separately and take rings off before rough work.
It breaks with two cleavages near 90°, which is worth knowing before you set it or drop it.
Skip the ultrasonic. Lukewarm water, mild soap and a soft cloth handle routine cleaning without risk.
A deeper look.
Extended detail on composition, sourcing, authentication and market value.
The physical record
The formula on record is (Na,Ca)(Al,Si)₄O₈ (An₁₀ to ₃₀). Crystal system: Triclinic. Specific gravity: 2.64 to 2.66. Streak: White. Cleavage and fracture: Two cleavages near 90°.
How it reaches the market
Granites and metamorphics worldwide; sunstone oligoclase from India and Norway; peristerite from Ontario (Canada).
What it gets confused with
Orthoclase: Potassium cousin, oligoclase shows fine parallel twin striations on cleavage that orthoclase lacks. Quartz: No cleavage flash, glassier. Albite: Continuous series, labs draw the line; field names follow the deposit.
What it is worth
Common; gem sunstone grades are mid-range.
Where buyers get caught
Goldstone glass impersonates its sunstone form constantly, uniform glitter means glass.
One more thing
Most affordable glittery sunstone is oligoclase with hematite confetti, Oregon's copper-bearing version is a labradorite cousin instead.
An education-first guide.
We do not carry oligoclase. This page exists because the question comes up and the honest answer is worth writing down, whether or not there is anything here to sell you.
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