igneous
Amygdaloidal Basalt
Amygdaloidal Basalt (containing secondary minerals like Quartz, Zeolites, or Calcite)
Hardness: 5-7 Mohs; Color: Dark grey to greenish-black groundmass with light-colored circular or almond-shaped inclusions; Luster: Dull (matrix) to vitreous (vesicle fill); Crystal structure: Aphanitic (fine-grained) matrix with amygdules.
- Hardness
- 5-7 Mohs
- Luster
- Dull (matrix) to vitreous (vesicle fill)
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Physical properties
Hardness: 5-7 Mohs; Color: Dark grey to greenish-black groundmass with light-colored circular or almond-shaped inclusions; Luster: Dull (matrix) to vitreous (vesicle fill); Crystal structure: Aphanitic (fine-grained) matrix with amygdules.
Formation & geological history
Formed from the cooling of mafic lava flows. As gas bubbles (vesicles) were trapped in the solidifying rock, they were later filled by secondary minerals precipitating from groundwater. Most common in ancient volcanic provinces (spanning from Precambrian to Cenozoic).
Uses & applications
Used primarily as decorative garden stones or aquarium decor. Exceptionally well-preserved specimens can be used for lapidary work (cabochons). Historically, when containing native copper (as in Michigan), used for metal extraction.
Geological facts
The name comes from the Greek word 'amygdala', meaning almond, referring to the shape of the mineral-filled gas bubbles. These rocks often trace the ancient migration path of hydrothermal fluids through volcanic fields.
Field identification & locations
Identify in the field by looking for a dark, heavy volcanic base pitted with lighter, rounded mineral 'spots' that don't look like standard crystals. Found commonly in Lake Superior (Michigan/Ontario), Iceland, and the Deccan Traps in India.
