thats really cool.
the cork + some coarse and fine sand should be great for the loose volcanic rocks on the right of the picture, but the bit i really like is the flowy piece on the left...
that looks a bit harder to do. maybe a big blob of thick PVA in teh right shape - allow the outside to harden and while the middle is still soft, manipulate it a bit with a toothpick or needle to get some interesting shapes (you can even prick it when the outside is hard and the inside is still liquid to get some oozing from the cured outer shell.
its harder to describe than i thought
though its a great idea. im assuming theyll be in the same colour grey?