concrete

The imposter that passes for conglomerate. Look at the aggregate: perfectly round, suspiciously even in size, because it was bought by the yard instead of laid down by a river. The cement between is one uniform gray, and sometimes a rust stain betrays the rebar that used to live in it.

Surf-polished concrete off an old pier can be genuinely handsome, and plenty of shelves hold a piece. Just know what you are holding. Real conglomerate has pebbles of many sizes and many rocks, in a cement that changes across the stone.