"...can get rocks with which to learn..."

I once read that porcelain is an acceptable substitute for obsidian when practicing, you might look around for an old broken toilet and what a chuck off of it.

We have a 55 pound chunk of pure black obsidian at home, once debated breaking a piece off to work with, but just couldn't do it.

If you are anywhere near central CA or western NV, there is a place, called Obsidian Dome, near June Lake. Not pure obsidian, it has what looks like granite in it, but you can take all you want...