砀 originally meant a stone with colored veins. Today it is met mostly in place names: 砀山 county in Anhui, renowned for its crisp pears, and the ancient 芒砀 hills where, by legend, the future Han founder Liu Bang hid as an outlaw.
The stone radical on the left, standalone 石 (stone). It gives the meaning: 砀 is a kind of veined rock, so it belongs with mineral and stone characters like 碎 (to smash) and 矿 (ore).
Supplies the sound. This is the old form behind 阳 (sun, bright); here it is reduced to a three-stroke residue and points to the dàng reading, with a faint flavor of brightness fitting a stone with shining veins.
