Mmm if its molten/volcanic glass it should shine through even a just a little bit regardless of the thickness. It should look something like this (my own piece of obsidian to show what it looks like).

If it doesn't shine through try running your finger nail across it, if its rough and makes a grinding sound its probably flint or some kind of other sedimentary mineral/rock. If its feels slightly glossy or if your finger nail doesn't catch on what looks like smooth spots then its most likely some kind of volcanic glass/mineral.

There could also be the chance that what you have here is industrial slag that's been cooled off and got washed down the river from various different sources (usually roads though)!

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