Andesite is a fine-grained volcanic rock intermediate in composition between basalt and rhyolite. It frequently exhibits a porphyritic texture, with distinct larger crystals of plagioclase or amphibole embedded in a finer groundmass. Collectors look for this rock primarily within subduction zone volcanic chains where it forms significant lava flows and volcanic edifices.
Is this andesite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch andesite with a known reference. Andesite sits at Mohs 6-7, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Andesite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Andesite typically shows a dull luster.
- 4Match the color rangeCompare against the expected color range: gray, dark gray, brownish-gray, greenish-gray.
- 5Look at form & habitTypical habit: aphanitic to porphyritic.
Often confused with
Andesite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside andesite
Minerals reported to co-occur with andesite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Mohs hardness
- 6-7
- Density
- 2.1-2.8 g/cm³
- Streak
- White
- Luster
- Dull
- Transparency
- Opaque
- Crystal habit
- Aphanitic to Porphyritic
- Cleavage
- None
- Rarity
- Common
- Uses
- Construction Aggregate, Road Metal, Decorative Stone
- Host rock
- Volcanic Arcs
- Typical price
- $1-10 per specimen for typical hand samples
Where rockhounds find andesite
2 mapped spotsClassic worldwide localities
- Andes Mountains, South America
- Cascade Range, USA
- Japan
- Indonesia
- New Zealand
Field-hunting tip
Look in volcanic arcs country, the host setting where andesite typically forms. If you start seeing plagioclase, pyroxene, hornblende in float, you are in the right ground. Field specimens usually show a aphanitic to porphyritic habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Oregon, Utah. Start trip planning there.
Common questions
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Find andesite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
