as long as it is rounded there is no problem. I have several tanks with sand on one side and gravel on the other and the rays seem to like either side equally. But, a new ray or a young ray may take comfort easier in sand because they can burr themselve's quicker.
Also when choosing sand make sure it fresh water safe, along with being safe for scaleless fish. Estes brand sand I know is deffinately safe for rays.
And! if you have any filter intakes going into your tank make sure you cover the intake with a sponge or some type of pre-filter to stop the sand from getting sucked up into the filter's motor.
And another note: If you have heaters inside your tank make sure you get heater guards for them, your rays WILL burn themselves on an unprotected heater.