On this occasion, however, I'm stumped!
It looks like it contains filaments/fibres, which can occur in rocks/minerals, but not really like that.
I get the feeling it may be man-made, perhaps waste glass-fibre and resin (from GRP manufacture), but that's just a guess. It might be useful to try a scratch test for hardness.
I'd also see if it floats or sinks in water, as a rough guide to density. If it floats, it could have come from a long way away; if not, it might have come from a local industry (but could still have come from far away if part of a wrecked cargo).