inferior variety of gem
玕 refers to a lower-quality or inferior type of gemstone, not a precious or flawless jewel.
玕 combines the jewel radical 王 with the phonetic component 干, which likely contributed the sound. The exact historical development is uncertain, but the structure suggests a phono-semantic compound for a type of gem.
The jewel radical 王 on the left shows it is a gem, but the right side 干 (dry) suggests it is a dry, inferior gem—not the best quality.
For カン, imagine a can of inferior gems being sold cheaply: can -> カン, and the can holds the low-quality 玕 stones.