igneous
Porphyritic Basalt (Amygdaloidal)
Porphyritic Mafic Volcanic Rock (Amygdaloidal Basalt)
Hardness: 5-6 (Mohs scale); Color: Dark reddish-brown (russet) matrix with white phenocrysts/inclusions; Luster: Dull to earthy (matrix), vitreous to pearly (inclusions); Structure: Porphyritic and vesicular; Specific Gravity: 2.8-3.0.
- Hardness
- 5-6 (Mohs scale)
- Luster
- Dull to earthy (matrix), vitreous to pearly (inclusions)
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Physical properties
Hardness: 5-6 (Mohs scale); Color: Dark reddish-brown (russet) matrix with white phenocrysts/inclusions; Luster: Dull to earthy (matrix), vitreous to pearly (inclusions); Structure: Porphyritic and vesicular; Specific Gravity: 2.8-3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. The white spots are amygdules (secondary minerals like quartz, calcite, or zeolites) that filled gas bubbles (vesicles) after the rock solidified. Common in regions with ancient volcanic activity like the Lake Superior basin or the Pacific Northwest.
Uses & applications
Commonly used as crushed stone in construction, road base, and railroad ballast. Smooth river or beach-tossed specimens are popular in rock tumbling and landscaping.
Geological facts
This specific reddish hue often indicates 'oxidation' of iron minerals within the basalt, sometimes referred to as 'ferruginous basalt.' When the white inclusions are large and distinct, these are colloquially known as 'snowflake' or 'polka-dot' rocks by collectors.
Field identification & locations
Identify in the field by its distinctive 'spotted' appearance against a dark, fine-grained background and its heavy weight relative to its size. Commonly found on volcanic beaches and in glacial till in the Northern US and Canada. Often confused with porphyry or certain types of rhyolite.
