砂 and 沙 are often interchangeable for 'sand,' but 砂 emphasizes coarser granules or industrial/technical uses (砂纸 sandpaper, 砂锅 clay pot, 砂糖 granulated sugar), while 沙 is the general word (沙滩 beach, 沙漠 desert).
Stone on the left is literal — sand is simply rock broken down into tiny grains. The radical marks 砂 as a mineral substance and groups it with 矿, 砖 and 砾 in the family of stones, ores and aggregates.
Provides the sound, drifting from shǎo to shā. The few / small meaning reinforces the sense too: each grain of sand is individually small, which makes 石 + 少 a memorable pairing — small bits of stone — and a near-twin of 沙 with a water radical.
