I don't suppose...

any of my Blipmates are industrial geologists/archaeologists(sp?)
I've been collecting these 'things' on my local beach for a few years now and since I'm displaying some art work soon which was inspired by them, it'd be really useful to be able to say what they are.
They're usually around apple sized and have obviously been through some kind of forming/heating/cooling process which has distorted the material.Some are less deformed and resemble the bottoms of vessels/bottles? They look like melted stone/glass.
The beach was next to an iron works, a chemical works, a glass works and a colliery.
I've had them to the National Glass Centre but no one there was able to say what they might be.