A dark, fine-grained igneous rock that is chemically similar to basalt but has a different crystal structure due to slower cooling; common in sills and dykes.
From French dolérite, ultimately from Greek dolos (deceitful) + -ite (mineral/rock suffix). The deceptive nature refers to how easily it's confused with other dark igneous rocks.
Dolerite forms the Giant's Causeway in Ireland and similar columnar basalt formations worldwide—the way it cools creates perfect hexagonal columns that look almost architectural, deceiving people into thinking giants or ancient engineers built them.
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