kind of jade
瑭 refers to a specific kind of jade, a precious ornamental stone. The meaning is narrow and concrete, centered on this particular type of jade.
瑭 combines 王 (jade) with 唐, which likely provides the sound. The character was created to name a specific type of jade.
The left side 王 (jade) tells you it's a precious stone, and the right side 唐 suggests the sound. Picture a Tang-dynasty artisan carving a unique piece of jade: that's 瑭.
For トウ, imagine a toe ring made of this special jade: toe -> トウ, and the jade ring adorns the toe.