igneous
Diabase (Dolerite)
Diabase or Dolerite
Hardness: 6-7 on Mohs scale; Color: dark grey to black with light speckling; Luster: sub-metallic to dull; Crystal structure: fine to medium grained ophitic texture where lath-shaped plagioclase crystals are embedded in pyroxene; Cleavage: none (conchoidal fracture); Specific gravity: 2.7-3.3
- Hardness
- 6-7 on Mohs scale
- Color
- dark grey to black with light speckling
- Luster
- sub-metallic to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: dark grey to black with light speckling; Luster: sub-metallic to dull; Crystal structure: fine to medium grained ophitic texture where lath-shaped plagioclase crystals are embedded in pyroxene; Cleavage: none (conchoidal fracture); Specific gravity: 2.7-3.3
Formation & geological history
Formed through the cooling of mafic magma in shallow intrusive bodies like dikes and sills. It is chemically similar to basalt but has a coarser grain size due to slower cooling underground. Often associated with the Triassic-Jurassic boundary (approx. 200 million years ago) in the Newark Supergroup.
Uses & applications
Primarily used as crushed stone for road construction and railroad ballast known as 'trap rock'. Higher quality specimens are used for monuments, headstones, and as ornamental stone for pool tables or counter tops.
Geological facts
The famous Stonehenge trilithons in England are made of a variety of dolerite often called 'bluestone'. In North America, the Palisades along the Hudson River are one of the most famous diabase formations.
Field identification & locations
Identify in the field by its distinctive 'salt and pepper' appearance caused by white plagioclase laths against dark pyroxene. It is heavier than most common rocks (dense) and often forms vertical columnar joints in large outcrops.
