Basalt is a dark-colored, fine-grained, igneous volcanic rock which is hard and dense. Individual mineral crystals are typically hard to see with the naked eye, although some kinds can contain larger crystals, or it may have holes that are empty (bubbles) or filled with other minerals.  Basalts are generally black or dark grey and polish well.  As a result, they are not easily scratched by metal objects. Basalt is often used for paving and ocasionally for wall construction.

Stone masonry building - walls are likely to be a mix of basalt and trachyte stones with variation in block size and colour (J. Bothara)

A stone masonry building built in 1920s in Christchurch, New Zealand; exterior stonework is Halswell basalt with Oamaru limestone facings (W. Clark)

A 19th century stone masonry construction: exterior wall wythe is fine grained grey Halswell basalt, with facing stones and base course of pinkish-brown Port Hills trachyte and string courses of speckled Hoon Hay basalt; Canterbury Provincial Buildings complex, Christchurch, New Zealand (W. Clark)