Andesine-Labradorite Identification Guide
How to identify andesine-labradorite, a plagioclase feldspar straddling two compositions, by feldspar tests, possible schiller, and treatment clues.
Read the full Andesine-Labradorite encyclopedia entry →
What Andesine-Labradorite Looks Like
"Andesine-labradorite" is a trade and compositional label for plagioclase feldspar near the boundary between andesine and labradorite (around An45–An55). The gem trade uses it for transparent feldspars of red, orange, golden, or green color, while in rocks the mineral is gray, white, or dark gray, sometimes showing a blue-to-green schiller (labradorescence) characteristic of labradorite. Luster is vitreous to pearly on cleavage faces.
Because it sits between two named species, its physical properties overlap both; identification relies on the general plagioclase toolkit plus density/optics for the exact composition.
Step-by-Step Field-ID Checklist
- Confirm feldspar — blocky habit, flat reflective cleavage surfaces, vitreous luster.
- Find two cleavages near 90° to confirm the feldspar group.
- Check for twinning striations on a cleavage face — confirms plagioclase over alkali feldspar.
- Tilt for schiller — a blue/green metallic sheen indicates the labradorite end of the range.
- Scratch test — Mohs 6–6.5; scratches glass, resists a knife.
Key Diagnostic Tests
- Mohs hardness: 6–6.5.
- Streak: white.
- Cleavage: two good cleavages at ~94°.
- Specific gravity: ~2.68–2.70 (slightly higher toward the labradorite end).
- Plagioclase twinning striations present.
- Inert to acid, non-magnetic.
Common Look-Alikes
- Pure andesine or pure labradorite: the same series; only precise composition (via optics, SG, or lab analysis) distinguishes them, which is exactly why the hyphenated term is used.
- Alkali feldspar: no twinning striations; sometimes shows adularescence (moonstone sheen) rather than the blue labradorescence.
- Sunstone: shows copper/hematite aventurescence (metallic glitter) rather than a sheet-like schiller.
- Quartz: Mohs 7 and no cleavage; feldspar shows flat cleavage planes.
- Treated red gems: uniform red/green transparent material may be copper-diffusion treated; surface-concentrated color is a clue.
Where It Is Found
Plagioclase of this composition is a rock-forming mineral in intermediate-to-mafic igneous rocks (diorite, gabbro, basalt, anorthosite). Labradorescent material famously comes from Labrador (Canada), Finland (spectrolite), and Madagascar. Transparent gem andesine-labradorite has been marketed from China, Tibet, Mongolia, and Congo, with the same treatment-disclosure concerns as gem andesine.
Frequently asked questions
How can you tell if it's andesine-labradorite?
Confirm plagioclase feldspar: Mohs 6–6.5, two cleavages near 90 degrees, and parallel twinning striations. The exact andesine-labradorite composition is pinned down only by optical or density testing, which is why the combined name is used.
What does andesine-labradorite look like?
In rocks it is gray-to-white blocky feldspar, sometimes flashing blue-green schiller; as a gem it appears as transparent red, orange, golden, or green faceted stones.
Andesine-labradorite vs labradorite — what's the difference?
They are adjacent members of the plagioclase series. Andesine-labradorite straddles the boundary (around An45–55), while labradorite is more calcium-rich; the labradorite end is more likely to show strong blue-green schiller.
Is andesine-labradorite gem material treated?
Often yes. Much transparent red and green gem material has been copper-diffusion treated. Look for color concentrated near the surface and request disclosure and certification for valuable stones.