Having luck with obsidian

Aluminum oxide pretty much stays whole, starts out sharp and rounds off. SiC is much more aggressive since it shatters into real sharp fragments, so they say.
AO does not do near as well for hard material like agate.
Never used AO as 220 stage for tumbling rocks, but have used AO 600 for step 3 on agate and it does fine. Most tumbling polish is AO.

Not to sure about the chemistry, they use a bunch of heat to make it. Details, like you say, if it cuts-use it.

I have tumbled a good bit of glass. The safety glass used on table tops rounds quickly. That blue glass is really hard, slow to round off.
Down right stubborn. It did hold it's own for filler.
Glass another chemistry mystery I suppose. The obsidian seems to be on the soft side.
I know red glass is most expensive, apparently they use gold to color it, yellow also as they use silver to color it.
Hard to find red glass. Wife sees it in the thrift store and buys it. Been stashing it.

Some glass has those flaws, guessing it has to do with cooling.
The old Mt Palomar Hale telescope was a 200 inch chunk of glass and they cooled it for months to avoid cracks.
They used borosilicate glass, whatever that is, sounds hard.
But am certain some obsidian cooled too fast and caused fractures/flaws.
You old school guys had kahunas to attempt such projects(the Hale lens).

Those tiny bubbles have to be gas pockets. They gotta be the culprits that release pressurized gas too.
The first few days of fresh coarse grit makes the most gas, guess the bubbles are getting opened faster.

Hoping to graduate to the polished obsidian club. Should know in about 20 days.