Hermit Crab Substrate

I am thinking about getting some hermit crabs but I cannot decide which substrate to use. I want to create a very natural looking environment for them because it will be used in a high school classroom. I would really like to use sand such as calci-sand because I think it looks more naturalistic but I want to use what is best for the crabs. I cannot decide whether to use sand or the coconut fiber bricks. I know that the bricks are good for humidity but what about calci-sand. Does it hold moisture fairly well?

I've never used sand so can't compare, but my Hermit crabs seem to be doing nicely in the bed-a-beast brick. They dig down and it does hold humidity very well. I have a calcium bowl (little milk cap) in the tank, along with a salt water bowl and regular water bowl. Hope that helps you a bit anyway...