grind, sharpen, mill
礱 centers on using friction to shape or process something: grinding, sharpening, or milling. All three meanings involve rubbing a surface against a hard material to refine or reduce it.
礱 combines 龍 (dragon) and 石 (stone). The stone component suggests grinding or sharpening with a stone, while 龍 likely contributes the sound. The exact historical development is uncertain.
A dragon 龍 sits on top of a stone 石, using its rough scales to grind and sharpen the stone into a millstone.
For ロウ, imagine a low grinding sound as the dragon works the stone: low -> ロウ.