igneous
Basalt (Water-worn pebble)
Mafic Igneous Rock (primarily Plagioclase feldspar and Pyroxene)
Hardness: 5-6 Mohs; Color: Dark grey to charcoal; Luster: Dull/Matte; Crystal structure: Aphanitic (fine-grained, crystals not visible to eye); Specific gravity: 2.8-3.0; Cleavage: None (conchoidal fracture).
- Hardness
- 5-6 Mohs
- Color
- Dark grey to charcoal
- Luster
- Dull/Matte
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Physical properties
Hardness: 5-6 Mohs; Color: Dark grey to charcoal; Luster: Dull/Matte; Crystal structure: Aphanitic (fine-grained, crystals not visible to eye); Specific gravity: 2.8-3.0; Cleavage: None (conchoidal fracture).
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. This specific specimen shows rounding and smoothing indicative of fluvial or coastal erosion (water-worn).
Uses & applications
Used extensively in construction as aggregate (crushed stone), for paving, and in the manufacturing of stone wool insulation. Dense specimens are also used for 'hot stone' massage therapy.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also found extensively on the Moon (the dark 'Maria') and Mars.
Field identification & locations
Identify by its dark color, heavy weight for its size, and lack of visible crystals. Commonly found in volcanic regions, riverbeds downstream from volcanic plateaus, and along beaches. Collectors value specimens with unique shapes or vesicles.
