Identification guide · Igneous Rocks
How to Identify Anorthosite
Anorthosite is usually white, gray, and blue gray with a pearly luster and is opaque. It has a Mohs hardness of 6. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. Its streak is white to pale gray. It is most often confused with Microcline, Albite, and Feldspar; the tests below separate them.
Anorthosite identification checklist
| Color | White, Gray, Blue gray, Black, Pale pink | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Pearly | Compare a fresh surface with glass and metal; this luster is softer than either. |
| Transparency | Opaque | Hold a thin edge up to a bright light and note whether light passes through. |
| Mohs hardness | 6 | It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. |
| Streak | White to pale gray | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | Variable by feldspar grains | |
| Fracture | Uneven to granular | |
| Specific gravity | 2.75 | Ordinary rock-like heft (specific gravity about 2.75), similar to quartz. |
Anorthosite look-alikes and how to tell them apart
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Anorthosite vs Microcline →
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Anorthosite vs Albite →
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Anorthosite vs Feldspar →
Both sit near Mohs 6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
Field tips
- Look for coarse feldspar-rich texture with mostly white, gray, or bluish plagioclase.
- Anorthosite usually has little visible quartz compared with granite.
- Dark patches can be pyroxene, ilmenite, magnetite, or related minerals.
- Fresh broken surfaces show feldspar grains better than weathered surfaces.
Anorthosite identification FAQ
How can you tell if a rock is Anorthosite?
Check the color (white, gray, blue gray, and black), the pearly luster, a Mohs hardness of 6 and a white to pale gray streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Anorthosite?
Anorthosite has a Mohs hardness of 6. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will.
What is the difference between Anorthosite and Microcline?
Anorthosite (Mohs 6, pearly luster) and Microcline (Mohs 6, vitreous luster) look alike, but hardness, streak and heft usually separate them. See the full Anorthosite vs Microcline comparison for a side-by-side table.
Is Anorthosite worth anything?
Entry-level Anorthosite specimens sell for around $1, typical collector pieces about $10, and premium examples $75 or more depending on size, quality and locality.
What color is Anorthosite?
Anorthosite is most often white, gray, blue gray, and black. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
