The black tahitian moon sand is very fine and very light, in weight. I've used it in the past and it didn't bother my rays bellies, but if you have strong filtration, which you should for a ray tank, it'll get sucked into your filter/s and cause serious problems.
I couldn't keep it from getting pulled into my FX5 and therefore I switched to the "caribsea torpedo beach sand", which is alot heavier and more course. I have to say it's a much better option than the black stuff.
Here's a link.
http://www.thatpetplace.com/Products/KW/torpedo/Class//T1/F32BX+0532+0069/EDP/50936/Itemdy00.aspx