This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
磌 combines 石 (stone) with 真, which likely contributes the sound. The exact historical development is uncertain.
The left side is stone 石, and the right side is 真 (truth). Picture a stone tablet carved with truth, but its exact meaning remains a mystery.
For テン, imagine ten stone tablets lined up: ten -> テン, each one a 磌.