In the book Sand Bed Secrets, Dr. Shimek states that virtually any material will work as long as it is of the correct size. The animals and the bacteria simply don't care much about the composition of the sand bed, they will burrow through and grow on just about everything. This is an excellent book by the way, a quick read at only 31 pages, but you'll know a lot about DSBs in roughly 30 to 45 minutes depending on how fast you read! Happy Holiday's.
Myk.