Tievebulliagh is formed from a volcanic plug, the intense heat generated by molten basalt has given rise to the formation of a durable flint, porcellanite, which is found at the foot of the eastern scree slope of the mountain. Three small outcrops of porcellanite can be seen on the higher south-east slope.
Flakes, rejects and part-finished axes can still be found round the hill and peak. It was here that the axes were roughed out before being finished at the sea shore. They were then exported as far afield as south-west Ireland, south-east England and north-east Scotland. No finished axes have been found, only unfinished and rejected ones.