Sand is a mix of two types of particals, generally referred to as platy and angular.
Platy particles tend to be rounder and are more prevelant in sands that have been "worked" ie tumbled long enough to knock the rough edges off. Where do you find it? At your local beach is probably the best place.
Angular particles are rougher and have sharper edges. They tend to be less soluble and are more prevalent in quarried sand. Masons and brick layers are familiar with bank sand (more platy materials) and sharp sand (more angular). The right mix of these sands is important when making mortars and concrete.
In aquariums the sharpness of the sand has a direct impact on how densely compact the substrate layer will become and how dificult it will be to clean. Your better off to use either a clean bank sand or beach sand mixed with a fine crushed coral to ensure good microbe populations and water quality.
If the rays get sore tummys then maybe someone can knit a little sweater for them?