[Zapmeister]

Any other gem cost would be just a random barrier.

I disagree. Someone pointed out earlier that a clam is a growing thing, so nature would not be inappropriate.

The problem with astral is that R'lyeh, the most obvious clam-hoarding candidate, has water/astral mages straight out of the box. Also, astrals are output by the clams, so the feedback loop is fed rather than inhibited.

Adding a small astral cost to clams would do little to slow down hoarding, IMHO.