igneous
Amygdaloidal Basalt
Amygdaloidal Basalt (Ca-rich Plagioclase, Pyroxene, and Olivine)
Hardness: 6-7 on Mohs scale; Color: Dark grey to black matrix with white or orange circular spots (amygdules); Luster: Sub-metallic to dull; Crystal structure: Aphanitic (fine-grained) matrix with secondary mineral fillings; Specific Gravity: 2.8–3.0.
- Hardness
- 6-7 on Mohs scale
- Luster
- Sub-metallic to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey to black matrix with white or orange circular spots (amygdules); Luster: Sub-metallic to dull; Crystal structure: Aphanitic (fine-grained) matrix with secondary mineral fillings; Specific Gravity: 2.8–3.0.
Formation & geological history
Formed from the rapid cooling of basaltic lava. The 'amygdules' (filled gas bubbles) form when groundwater later deposits minerals like calcite, quartz, or zeolites into the vesicles. Common in volcanic regions and Cenozoic age deposits.
Uses & applications
Used primarily in construction (crushed stone), as a decorative stone in landscaping, or as geological teaching specimens for studying volcanic processes.
Geological facts
The word 'amygdule' comes from the Greek word for 'almond,' referring to the shape of the mineral-filled gas bubbles. Many of the world's finest agates and zeolites are found as fillings within this type of basalt.
Field identification & locations
Identify by looking for a heavy, dark, fine-grained rock with distinct white or colored mineral 'pockets' that are not part of the original cooling crystals. Commonly found in the Lake Superior region, Hawaii, and Iceland.
