Identification guide · Igneous Rocks
How to Identify Andesite
Andesite is typically gray, greenish-gray, or brownish, with a fine-grained, aphanitic groundmass that may enclose visible white plagioclase or dark hornblende phenocrysts. Its overall hardness runs around 6 on the Mohs scale, reflecting the feldspar and pyroxene it contains, though testing individual mineral grains gives a better read than testing the rock as a whole. Streak is not diagnostic since it is a rock rather than a single mineral. Andesite is frequently confused with basalt, which is darker and more iron-rich, and with fine-grained diorite, which cools underground rather than erupting.
Andesite identification checklist
| Color | Gray, Dark gray, Greenish gray, Brown, Black | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Dull groundmass with vitreous phenocrysts | Fresh surfaces shine like glass; a dull surface usually means weathering, so chip or wet a corner. |
| Transparency | Opaque | Hold a thin edge up to a bright light and note whether light passes through. |
| Mohs hardness | 5–6 | It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. |
| Fracture | Conchoidal to uneven | Broken surfaces show smooth, curved, shell-like scoops. |
| Specific gravity | 2.5–2.8 | Ordinary rock-like heft (specific gravity about 2.65), similar to quartz. |
Extra tests for Andesite
Glass check. Andesite shares a glassy luster and conchoidal fracture with man-made glass. Glass sits near Mohs 5.5 and often contains round gas bubbles, so a hardness test and a hand lens separate them.
Andesite look-alikes and how to tell them apart
Quartz is harder (Mohs 7) than Andesite (Mohs 5–6), so Quartz will scratch Andesite and not the other way round. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. Andesite vs Quartz →
Both sit near Mohs 5–6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Andesite vs Gabbro →
Both sit near Mohs 5–6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Andesite vs Obsidian →
Both sit near Mohs 5–6, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
Andesite identification FAQ
How can you tell if a rock is Andesite?
Check the color (gray, dark gray, greenish gray, and brown), the dull groundmass with vitreous phenocrysts luster, a Mohs hardness of 5–6. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Andesite?
Andesite has a Mohs hardness of 5–6. 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 Andesite and Quartz?
Andesite (Mohs 5–6, dull groundmass with vitreous phenocrysts luster) and Quartz (Mohs 7, vitreous, greasy on fracture surfaces luster) look alike, but hardness, streak and heft usually separate them. See the full Andesite vs Quartz comparison for a side-by-side table.
Is Andesite worth anything?
Entry-level Andesite 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 Andesite?
Andesite is most often gray, dark gray, greenish gray, and brown. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
