How big is big? What you need to make sure of is that you can evenly distribute the weight. If we're talking a 20-30+lb rock, make sure it has a nice flat spot that it can sit on a little glass with. If it's not possible to sit it on a flat edge, egg crate on the bottom might help with that. I normally don't recommend eggcrate for lots of smaller rock, since it's unnecessary IMO
... but for larger rocks, then it can be of value.
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