Basalt
Basalt is a dark, fine-grained volcanic rock that makes up most of the ocean floor and Hawaiian Islands. It often forms distinctive columnar jointing.
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Basalt properties at a glance
| Type | Igneous Rocks |
|---|---|
| Chemical formula | Variable mineral composition |
| Mohs hardness | 6 |
| Luster | dull |
| Streak | Variable; test individual minerals |
| Cleavage | Variable by mineral grains |
| Fracture | Conchoidal |
| Specific gravity | 3 |
| Transparency | opaque |
| Common colors | Black, Dark Gray, Dark Green |
| Rarity | common |
Geology & background
Forms from rapid cooling of low-viscosity lava at Earth's surface, creating fine-grained texture.
Used in construction since ancient times; the Rosetta Stone is carved from basalt.
How to identify Basalt
- Check whether the specimen's color and texture match typical Basalt examples.
- Photograph a fresh surface in natural light and avoid shadows, glare, or wet surfaces.
- Use hardness, streak, luster, grain size, and locality together; no single trait is enough for every specimen.
Where Basalt is found
Basalt is reported from multiple localities or geological settings. Exact locality should be verified from specimen provenance, maps, or field notes.
How much is Basalt worth?
| Entry-level specimen | $1 |
|---|---|
| Typical collector specimen | $5 |
| Premium specimen | $30+ |
Value depends on size, condition, color, crystal quality, locality, rarity, preparation, and provenance. Treat this as educational guidance, not an appraisal.
Common uses
- Construction aggregate
- Building stone
- Road base
Interesting facts
- Most common rock on Earth's surface
- Giant's Causeway in Ireland features basalt columns
- Mars has vast plains of basalt
Care & safety
Treat Basalt as a collector specimen unless hardness, cleavage, and fracture are confirmed.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a properly rated respirator.
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