igneous
Basalt with Quartz Infill
Aphanitic Mafic Igneous Rock with Silica (SiO2) inclusion
Hardness: 6-7 (Mohs); Color: Dark grey to black matrix with a milky white/translucent inclusion; Luster: Dull or earthy (matrix), Vitreous (quartz); Crystal Structure: Fine-grained aphanitic matrix; Specific Gravity: 2.8-3.0
- Hardness
- 6-7 (Mohs)
- Luster
- Dull or earthy (matrix), Vitreous (quartz)
Identified More igneous →
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Physical properties
Hardness: 6-7 (Mohs); Color: Dark grey to black matrix with a milky white/translucent inclusion; Luster: Dull or earthy (matrix), Vitreous (quartz); Crystal Structure: Fine-grained aphanitic matrix; Specific Gravity: 2.8-3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at the Earth's surface. The lighter inclusion likely formed as an amygdule, where silica-rich fluids filled a gas bubble (vesicle) post-eruption.
Uses & applications
Common basalt is used in construction as aggregate, road base, and railroad ballast. Smooth river-worn specimens like this are often kept as curiosities or used in landscaping.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. When weathered by water into smooth shapes, they are often called 'river stones' or 'beach pebbles'.
Field identification & locations
Identify by its dark, heavy, fine-grained texture and lack of visible crystals in the dark matrix. Its rounded shape indicates significant water erosion from a river or coastline.
