Thanks for the responses everyone! Pumice was what I was thinking as well. Most porosity, still pretty light, but doesn't move around in the soil. Expanded shale and pea gravel doesn't have enough pore space for my liking. Also pretty heavy because of this.
And I agree with everyone else, it is difficult to find. I'm embarrassed to say I paid $37 for 5 gallons once just to test it out (didn't seem like 5 gallons either). I absolutely loved it, but it was so damn expensive. I'm going to be setting up a larger vermicomposting bin and I'm looking for some aeration. Good chance it will end up no till, so I'm definitely looking for something longer lasting then perlite, and damn I hate the dust perlite makes! I like scoria a lot, but it is a little less porous, and a little heavier yet then pumice.
Greesemonkey, yes, they are moderately similar imo, with the exception of perlite. I've really grown to hate that stuff.
https://en.wikipedia.org/wiki/Lava_rock
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