igneous
Basalt Pebble
Aphanitic mafic extrusive igneous rock
Hardness: 6 on Mohs scale; Color: Dark brown or weathered reddish-brown on the exterior, dark gray to black on fresh fractured surfaces; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (microcrystalline or glassy, crystals too small to see with the naked eye); Cleavage: None; Specific Gravity: 2.8 - 3.0
- Hardness
- 6 on Mohs scale
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6 on Mohs scale; Color: Dark brown or weathered reddish-brown on the exterior, dark gray to black on fresh fractured surfaces; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (microcrystalline or glassy, crystals too small to see with the naked eye); Cleavage: None; Specific Gravity: 2.8 - 3.0
Formation & geological history
Formed from the rapid cooling of low-viscosity, iron- and magnesium-rich lava flows (basaltic lava) at or near the Earth's surface. This particular specimen has been naturally tumbled, rounded, and smoothed by water action in a stream or beach environment, and subsequently broken to reveal a fresh gray interior.
Uses & applications
Used in construction as crushed stone, railroad ballast, aggregate in concrete, and dimension stone, as well as geological and lapidary study.
Geological facts
Basalt is the most common rock type in the Earth's crust underlying the oceans and forms most oceanic islands like Hawaii. Weathering of basalt creates iron-rich clay soils.
Field identification & locations
Easily identified by its dark, dense, fine-grained matrix and water-worn smooth exterior. Found globally wherever volcanic activity has occurred and in associated riverbeds and shorelines.
