Amygdaloidal basalt is a volcanic rock characterized by vesicles that have been filled with secondary minerals. These filled cavities, known as amygdules, often contain high-quality zeolites, agate, or other silicates and are highly prized by collectors.
Is this amygdaloidal basalt?
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 amygdaloidal basalt with a known reference. Amygdaloidal Basalt sits at Mohs 5-6, softer than the next harder reference, harder than the previous one.
- 2Read the textureLook at a fresh break with a hand lens. Texture: Gas vesicles filled with secondary minerals (amygdules). Grain size: Fine-grained (aphanitic).
- 3Pick out the mineralsComposition: Basalt with amygdules of calcite, quartz, chlorite or zeolites. Identify the two or three most abundant grains.
- 4Read the lusterHold the specimen under a strong light. Luster: dull.
- 5Match the color rangeCompare against the expected color range: gray, black, green, brown.
Often found alongside amygdaloidal basalt
Minerals reported to co-occur with amygdaloidal basalt. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Mohs hardness
- 5-6
- Density
- 2.8-3.0 g/cm³
- Luster
- Dull
- Transparency
- Opaque
- Rarity
- Common
- Uses
- Collector, Geological study
- Host rock
- volcanic flows
- Texture
- Gas vesicles filled with secondary minerals (amygdules)
- Grain size
- Fine-grained (aphanitic)
- Composition
- Basalt with amygdules of calcite, quartz, chlorite or zeolites
Where rockhounds find amygdaloidal basalt
1 mapped spotsClassic worldwide localities
- Lake Superior region, USA
- Iceland
- Faroe Islands
- India
- Scotland
Field-hunting tip
Where it forms: volcanic flows. If you start seeing zeolites, quartz, calcite in float, you are in the right ground. In the U.S., the densest reported localities are in Michigan. Start trip planning there.
Common questions
How do you identify amygdaloidal basalt?+
Where is amygdaloidal basalt found?+
Can I find amygdaloidal basalt in the United States?+
What minerals are found with amygdaloidal basalt?+
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Find amygdaloidal basalt on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
