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The rock was found on the coast near the northern tip of the Isle of Bute in Scotland.
A small area has been cut and polished as per the photographs. It is also magnetic.
It weighs just under 2kg and measures approx 14x10x7cm.
Any thoughts would be grand!
It is a volcanic rock, probably basalt going on the dark and fine-grained nature (the fracture is not glassy enough for it to be obsidian), and on the Bute location.
Basalt contains sufficient iron (in the form of ferromagnesian minerals and magnetite) that it can be magnetic.
Much of northern Bute is composed of metamorphic rocks, but it contains numerous basic dykes, part of the Mull dyke-swarm, some of which are basalt. That is probably the origin of your specimen.
Most of the surface of your specimen is weathered broken surface, but an unbroken part shows flow, originating when the lava was mobile.