To test if it's aragonite sand, just put some in a cup with some vinegar, and see if the sand dissolves. If it dissolves, you're in good shape. I think I read that in a 'Southdown' FAQ on RC somewhere.
The Oldcastle that I bought at Chase Pitken last year passed the vinegar test, but it also said 'not for aquarium use'. I never got to try it in the tank, because it ended up in the sandbox. I was actually planning to use it in my 'fuge -- I like coarser substrate in the display tank...
Regards,