Mouldable clay substrate is almost the best thing since sliced bread! If used correctly it can give you the closest thing to a bearded dragon or leopard geckos natural substrate. It is also ideal for species that live in dry areas that need to burrow, we have used this substrate with desert hairy scorpions with great success.
So basically its a dry clay powder, you mix it with water, use it to mould out a substrate base with caves etc and it dries rock hard. If you want to give your bearded dragon the most naturally accurate substrate possible use excavator clay to cover the base of your vivarium and then when it has dried rock hard sprinkle a fine layer of red desert sand over the top.
For species that like to burrow make a landscape and start off entrances to caves etc and you will find your pet excavating Them to its own personal liking.