This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
磳 combines 石 (stone) with 曾, which likely contributes the sound. The exact historical development is uncertain.
The stone radical 石 suggests a connection to rocks or minerals, but the overall meaning is obscure. Picture a mysterious stone tablet with the 曾 component carved into it, its purpose unknown.
For ソウ, imagine a soul (ソウ) trapped inside the stone (石) of 磳, whispering its forgotten name.