igneous
Amygdaloidal Basalt
Basalt with secondary mineral infilling (SiO2/CO3)
Hardness: 5-6 (matrix), 6-7 (inclusions); Color: Dark grey to black matrix with reddish-orange or white circular spots; Luster: Dull to earthy matrix, vitreous inclusions; Crystal structure: Aphanitic (fine-grained matrix); Specific gravity: 2.8-3.0
- Hardness
- 5-6 (matrix), 6-7 (inclusions)
- Luster
- Dull to earthy matrix, vitreous inclusions
Identified More igneous →
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Physical properties
Hardness: 5-6 (matrix), 6-7 (inclusions); Color: Dark grey to black matrix with reddish-orange or white circular spots; Luster: Dull to earthy matrix, vitreous inclusions; Crystal structure: Aphanitic (fine-grained matrix); Specific gravity: 2.8-3.0
Formation & geological history
Formed from the rapid cooling of basaltic lava. As the lava cools, gas bubbles are trapped, creating cavities (vesicles). Over geological time, mineral-rich groundwater flows through, depositing minerals like quartz, agate, or zeolites into the holes.
Uses & applications
Used primarily as decorative specimens for collectors, educational purposes in geology, and occasionally crushed for high-quality road base or construction aggregate.
Geological facts
This specific piece shows 'amygdules' (from the Greek word for 'almond') that appear to be filled with reddish agate or chalcedony. It is famous for being the host rock of Lake Superior Agates in the United States.
Field identification & locations
Identify by looking for small, almond-shaped or circular voids filled with contrasting minerals in a dense, dark volcanic rock. Common in volcanic regions such as the Keweenaw Peninsula (Michigan), basaltic flows in Oregon, and the Deccan Traps (India).
