rock, cliff
矸 centers on solid, rocky terrain: a rock or cliff. The two meanings are closely related, both referring to stone formations.
矸 combines 石 (stone) with 干, which likely contributes the sound. The exact historical development is uncertain, but the stone component clearly points to a rocky meaning.
The left side is 石 (stone), and the right side is 干 (dry). Picture a dry, towering stone cliff rising from the ground.
For カン, imagine a can sitting on a rocky cliff: can -> カン, and the stone cliff holds the can.