a precious stone
琅 centers on a type of precious stone or gem, often with a clear, beautiful quality.
琅 combines 王 (jade, jewel) with 良, which likely contributes the sound. The character refers to a type of precious stone.
The jewel radical 王 beside 良 suggests a fine, beautiful stone. Imagine a king (王) holding a good (良) gem: that's 琅, a precious stone.
For ロウ, picture a row of precious stones lined up on a shelf: row -> ロウ, and each stone is a 琅.