Hi, our sand is the black tahitian sand, the blue spot jawfish has burrowed in it fine but we also have bits of larger substrate broken pieces of rock (rubble) that when we ordered our rock from Marine depot live there was plenty of, as of today he has taken some of that rock and has supported what he has dug, this fish is the coolest, they are a bit high but very fun to watch. I did a lot of research before getting him to find out what was needed, from what I have read they like sand but also like some rocky substrate. Here's some sites that have helped me :
go to enviromental conditions
but what has helped me with research with fish is this book which I bought through AA it's called The Conscientious Marine Aquarist, it's really helped us out on trying to figure out what kind of fish, how big, what they eat and so on.
Hope this helps