A recent answer stated that egg shell can be crushed and used as a calcium supplement in Do-it-yourself food. This got me relating it to crushed coral, which is commonly used as a substrate.
I've crushed one egg shell and I noticed they get very fine, and have a nice bright white color, they also are denser than water and so they sink down, as a substrate should. Not that I'd go through the elaborate trouble of crushing a big number of egg shells to make a substrate from them, perhaps industrially it'd be a good idea, since they're cheap and perhaps non-toxic paint can be applied for decoration. This is all basically theoretical, though it might be already applied without my knowledge. This also is a solution for pets that are known to swallow substrate and can be used as a mean of avoiding impaction.
Is it safe to use egg shell as a substrate for aquariums and terrariums?