Andesine Identification Guide
Identify andesine, an intermediate plagioclase feldspar, by its two cleavages, twinning striations, and hardness, and learn the treated red-gem controversy.
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What Andesine Looks Like
Andesine is a member of the plagioclase feldspar series, intermediate in composition (about An30–An50, between albite and labradorite). In rocks it is usually white, gray, greenish, or pale yellow, with a vitreous to slightly pearly luster and translucent-to-transparent appearance. As a gemstone it is sold in red, orange, and green transparent forms, some of which are diffusion-treated.
The diagnostic feature shared by all plagioclase is polysynthetic twinning: fine, parallel striations (grooves) on one cleavage surface that catch the light — a feature alkali feldspars lack.
Step-by-Step Field-ID Checklist
- Confirm feldspar — look for blocky grains with flat, reflective cleavage faces and vitreous luster.
- Find two cleavages meeting at about 90° (nearly right angles) — the feldspar signature.
- Look for twinning striations — fine parallel lines on a cleavage face confirm plagioclase rather than orthoclase/microcline.
- Scratch test — Mohs 6–6.5; scratches glass, not scratched by a knife.
- For gems, note color and ask about treatment — saturated red/green transparent andesine may be copper-diffusion treated.
Key Diagnostic Tests
- Mohs hardness: 6–6.5.
- Streak: white.
- Cleavage: two good cleavages intersecting at ~93–94° (close to right angles).
- Specific gravity: ~2.66–2.68.
- Twinning striations visible on cleavage faces — diagnostic for plagioclase.
- Inert to acid, non-magnetic.
Common Look-Alikes
- Alkali feldspar (orthoclase/microcline): lacks plagioclase twinning striations; microcline often shows a cross-hatched (tartan) pattern instead. Striations = plagioclase.
- Other plagioclase members (oligoclase, labradorite): distinguished only by composition (optics/SG); labradorite often shows blue-green schiller (labradorescence) that andesine usually lacks.
- Quartz: harder (Mohs 7), and crucially has no cleavage — only conchoidal fracture, versus feldspar's flat cleavage planes.
- Red gem andesine vs sunstone/oregon sunstone: natural copper-bearing labradorite-sunstone shows aventurescent copper platelets; uniform diffusion-treated andesine may show color concentrated near the surface.
Where It Is Found
Andesine is a rock-forming mineral of intermediate igneous rocks, giving its name to andesite, and is common in diorite and many volcanic and metamorphic rocks worldwide. Gem-quality transparent andesine has been marketed from Tibet, China, Mongolia, and the Democratic Republic of Congo, with the red/green Tibetan material being the subject of well-known treatment-disclosure debates.
Frequently asked questions
How can you tell if it's andesine?
Confirm it is plagioclase feldspar: Mohs 6–6.5, two cleavages meeting near 90 degrees, and fine parallel twinning striations on a cleavage face. Andesine specifically is the intermediate plagioclase, identified precisely only by optical or density measurement.
What does andesine look like?
In rocks it is white-to-gray blocky feldspar with flat shiny cleavage faces; as a gem it appears as transparent red, orange, or green faceted stones, some of which are color-treated.
Andesine vs orthoclase — how do I tell them apart?
Andesine is a plagioclase and shows fine parallel twinning striations on its cleavage faces; orthoclase is an alkali feldspar and lacks those striations. Microcline may show a tartan cross-hatch instead.
Is gem andesine natural or treated?
Both exist. Much saturated red and green transparent andesine has been copper-diffusion treated. Treated stones may show color concentrated near the surface; always ask for disclosure and lab certification for high-value pieces.