石 depicts a rock at the foot of a cliff - top stroke the cliff edge, 口 the stone below. As radical it places 砖 in the hard-material family alongside 矿砌碎碗 - bricks fired hard belong with stones.
专 supplies the sound zhuān directly. The traditional 磚 used 專 as phonetic - both forms give the same reading. The reform replaced the complex 專 with the simpler 专. The same phonetic is in 转砖传.
