A hard igneous rock (6)

I believe the answer is:

basalt

(Other definitions for basalt that I've seen before include "Black volcanic rock" , "Type of rock is a blast, oddly" , "Dark fine-grained volcanic rock" , "Dark rock of volcanic origin" , "Commonest type of solidified lava; dense, dark grey and fine-grained" .)

I've seen this clue in the Sydney Morning Herald.

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