A 璜 is half of a 璧 (the round jade disc). 《说文》 defines it as 'half of a bi.' It was used in royal court ritual - the 《周礼》 prescribes it as the jade for ceremonies honoring the north.
王 on the LEFT is actually the jade radical 玉 in its compressed form (the dot of 玉 is dropped when it sits on the left). It indexes jewelry and gemstone words - 玲，珠，玛，璧 - and here marks 璜 as a piece of carved ritual jade.
黄 supplies the sound huáng exactly, same tone. It may also hint at the yellow-amber colour of ancient ceremonial jade. 璜 is a semicircular jade pendant from the Zhou ritual set, paired with the round 璧 and the tube-shaped 琮。
