If you thought sand was originally dangerous, why would you even consider bark chips?
A.gigas is right on. The other comment is apparently uneducated. Calcium sand was invented for a good intention. Unfortunately, leopard geckos cannot digest large blobs of grainy calcium. Crushed walnut shells are just plain ridiculous, undigestable.
Now if you do want to add sand, do ensure that your geckos are healthy with a sufficient diet of vitamins and calcium, they are very important if you do not want the gecko eating the substrate.
Last thing. Do not trust pet shop labels.