Basalt, dolerite and gabbro are basic igneous rocks. The distinction between the three varieties of is based on grain size:
Basalt– fine-grained. Individual crystals are rarely visible to the naked eye
Dolerite– medium-grained. Crystals can be distinguished with the naked eye
Gabbro– coarse-grained. Individual crystals visible
The most common type of local igneous rock is basalt. Local basalt formed during the Tertiary (60-65 million years ago) and formed an extensive plateau that covers most of County Antrim and extends into Londonderry. Its most famous expression is at the Giant’s Causeway World Heritage Site. Local basalt is black and fine-grained often with visible vesicles (air bubbles which formed as the rock was cooling).
It is an extremely durable stone type although surface discolouration caused by oxidation of iron-rich minerals can give it a ‘rusty’ appearance. It is used as rock-faced blocks and rubble stone.