igneous
Amygdaloidal Basalt
Basalt with secondary mineral infilling
Hardness: 5-6 (matrix); Color: dark gray to greenish-black; Luster: dull to earthy; Texture: Vesicular (pitted) to amygdaloidal (filled pits); Structure: Fine-grained aphanitic matrix.
- Hardness
- 5-6 (matrix)
- Color
- dark gray to greenish-black
- Luster
- dull to earthy
Identified More igneous →
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Physical properties
Hardness: 5-6 (matrix); Color: dark gray to greenish-black; Luster: dull to earthy; Texture: Vesicular (pitted) to amygdaloidal (filled pits); Structure: Fine-grained aphanitic matrix.
Formation & geological history
Formed from the rapid cooling of basaltic lava. The holes (vesicles) are created by trapped gas bubbles during eruption. If these holes later fill with minerals like quartz, calcite, or zeolites due to groundwater infiltration, it is called 'amygdaloidal'.
Uses & applications
Common basalt is used in construction as crushed stone for roads and railroads. Polished specimens are used as decorative stones or for educational geological collections.
Geological facts
Many of the world's finest copper and agate specimens are found within the amygdaloidal cavities of ancient basaltic lava flows, such as those in the Lake Superior region.
Field identification & locations
Identify in the field by its dark color, heavy weight (high specific gravity), and the presence of small rounded cavities or white/colored mineral 'spots' (amygdules) within a dense matrix.
