precious stone, Xian in Sichuan
珙 centers on a precious stone or gem, with a secondary sense as a place name in Sichuan. The two meanings do not share an obvious conceptual link beyond the character itself.
珙 combines 王 (jade, jewel) with 共, which likely contributes the sound. The exact historical development is uncertain.
The left side 王 (jewel) and right side 共 (together) suggest a precious stone that brings things together, like a gem that completes a collection.
For キョウ, imagine a precious stone that makes a clear 'kyo' sound when tapped: kyo -> キョウ, like a crystal bell.