I agree with Chompers in that how sharp the sand is would be of little concern to rays. Sharp sand however will pack together and bridge more effectivley than bank sand. A good example can be seen on the macro particle level when looking at gravels. "Peastone" gravels are very round and walking in them you observe that the material readily moves out of the way and if your over 200lbs like me you can actually sink into them. Crushed gravels which are sharper than peastone pack well and form membranes that have some integrity. Anyone who has tried to dig through packed 7/8"-down (pit run) knows what I mean.
Packed sharp aquarium sand when bridged prevent oxygen from getting below the top of the sand layer and inhibits aerobic bacterial function. Anaerobic bugs can be established and the biproducts of their reactions include sulphur compounds and methane. You dont want either coming out of your substrate.
Using just angular sand grains (like oolytic sand) will result in clouds of sand developing in your tank every time a large ray makes a move. (Maybe this is a good thing but you wont be able to see their sweaters very well)
Make sure you have a good mixture of two parts bank sand to one part sharp sand. This should give the mix good structural properties without preventing water movement through the interstices of the sand bed.
