uncut gem, unpolished gem
璞 centers on a gemstone in its raw, natural state: uncut, unpolished, and still embedded in its original matrix. The idea is of something valuable but not yet refined.
璞 combines 王 (jade/gem) with 菐, which likely contributes the sound. The character originally referred to unworked jade, and the meaning extended to any uncut gem.
The left side 王 (jewel) sits beside the complex right side 菐, which looks like a rough, unshaped stone. Picture a raw gemstone still covered in its rocky crust, waiting to be polished.
For ハク, imagine a gem hunter hacking away at a rough stone to reveal the uncut gem inside: hack -> ハク, and the rough stone is the unpolished gem.