石 pictures a stone fallen at the foot of a cliff. The 厂 element on top is a cliff face seen from the side; the 口 below is the loose rock that broke off and rolled down. Identical across 繁體 / 新字体 / 简体.

As a radical, 石 marks anything stony, mineral, or hard: 砂 (sand), 砲 (cannon — stone-projectile weapon), 碑 (stele/stone tablet), 礎 (foundation stone), 確 (firm/certain — originally "as solid as stone"), 磁 (magnetic), 砂糖 (sugar — note the stone radical for crystalline substance). When 石 appears on the left, the meaning is almost guaranteed to involve rock, mineral, or hardness.

Mandarin: shí, rising 2nd tone. 石头 (shítou, stone/rock), 石油 (shíyóu, petroleum — "stone oil"), 化石 (huàshí, fossil — "transformed stone"), 钻石 (zuànshí, diamond), 宝石 (bǎoshí, gemstone). The character also serves as a common surname.

Japanese: on-reading セキ (seki) in 化石 (kaseki, fossil), 宝石 (hōseki, jewel), 石碑 (sekihi, stone monument). Less common on-reading シャク (shaku) survives in older readings. The kun-reading いし (ishi) is the everyday word — 石 (ishi, stone), 石橋 (ishibashi, stone bridge — also a common surname). Plus 小石 (koishi, pebble) and 石垣 (ishigaki, stone wall) — Okinawan castle architecture is famous for it.

Memory aid: a cliff overhang (the curved 厂 stroke) with a fallen rock (the 口 box) tucked underneath. Erosion in two strokes.