pile of stones
砢 centers on a heap or pile of stones, evoking a rough, rocky accumulation.
砢 combines 石 (stone) with 可, which likely contributes the sound. The exact historical development is uncertain, but the stone component clearly points to a rocky meaning.
The left side 石 means stone, and the right side 可 looks like a pile of stones stacked up. Together they form a heap of stones.
For ラ, imagine a pile of stones that makes a rough 'ra' sound as they tumble down: ra -> ラ.