硅

Left stone radical - a pictograph of a piece of rock fallen from a cliff (厂 above + 口 the loose chunk beneath). The indexing element. Anchors 硅 in the mineral family with 矿 (ore), 砂 (sand), 砖 (brick), 碳 (carbon). The radical is precisely chosen: silicon is the chief element of sand and quartz.

Right 圭 supplies the sound - guī matches the parent reading with no drift. 圭 depicts two squares of earth stacked, the classical ceremonial jade tablet held by nobles. Same phonetic appears in 闺 (boudoir), 桂 (cinnamon), 街 (street). For 硅 it functions as a pure sound-marker - a 19th-century coinage rendering the Western chemical element silicon.