Igneous Rocks · common
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.
Also known as: volcanic rock, giants causeway, black rock.
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.
Full Basalt identification guide: checklist, home tests and look-alikes →
Basalt vs similar stones
Not sure it isn't something else? Side-by-side comparisons with the stones most often confused with Basalt.
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.
Basalt FAQ
What is 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.
How much is Basalt worth?
Entry-level Basalt specimens sell for around $1, a typical collector specimen for about $5, and premium pieces for $30 or more. Value depends on size, condition, color, crystal quality, locality, rarity, preparation, and provenance. Treat this as educational guidance, not an appraisal.
How hard is Basalt?
Basalt has a Mohs hardness of 6. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will.
Where is Basalt found?
Notable sources of Basalt include Hawaii, Iceland, India, and Washington State, USA. Forms from rapid cooling of low-viscosity lava at Earth's surface, creating fine-grained texture..
What is Basalt used for?
Basalt is used for construction aggregate, building stone, and road base.
