# What is basalt?
**Basalt is a hard, dark grey to black volcanic rock formed when lava cools rapidly at the Earth’s surface.** It is one of the most common rocks on Earth and has been used as a building material for centuries due to its extreme hardness and durability.
## Properties
– **Very hard** — 6–7 on the Mohs hardness scale, extremely wear-resistant
– **Weather-resistant** — virtually unaffected by frost, rain, and chemical attack
– **High compressive strength** — can bear heavy loads without breaking
– **Dense and low porosity** — absorbs very little moisture
## Application
### Sea defences and revetments
Basalt blocks and columns have been used for centuries to protect dykes and embankments against wave action. The hexagonal basalt columns are particularly characteristic of Dutch flood defences.
### Paving
Basalt setts and cobbles provide durable surfacing for roads, squares, and driveways. The material is extremely abrasion-resistant and withstands heavy traffic.
### Aggregate
Crushed basalt (chippings) serves as aggregate in concrete and asphalt, providing higher compressive strength than limestone or gravel.
### Stone wool
Basalt is the raw material for stone wool (rock wool), a widely used insulation material in construction.
## Related terms
– Natural stone
– Rubble stone
– Stone wool
– Insulation
– Paving patterns
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