The majority of sand found in the world (like 99% or something) is made of quartz. White Sands is unique because it is formed from gypsum, which is rarely found in the form of sand because it is water-soluble. Normally, rain would dissolve the gypsum and carry it to the sea. Since the Tularosa Basin has no outlet to the sea, rain that dissolves gypsum is trapped within the basin, and the rain either sinks into the ground or forms shallow pools which subsequently dry out and leave gypsum in a crystalline form on the surface.