it looks like smooth rounded quartz-based stuff with other stuff in it

If it's quartz, it should be okay. But some of the gravels sold for fishtanks
are not inert. If they're meant for marine or African tanks, they can raise
the GH quite a bit.

no
fizzing when I put it into vinegar, etc).

Vinegar probably wouldn't be strong enough to get a reaction.



Leigh

http://www.fortunecity.com/lavender/halloween/881/