硝 (saltpeter/potassium nitrate) was a key ingredient in traditional Chinese gunpowder alongside 硫 (sulfur) and 炭 (charcoal). Ancient Chinese alchemists discovered the formula, making 硝 historically significant in world history.
石 (stone) is the indexing radical. 硝 names saltpetre (potassium nitrate) - a mineral crystal historically harvested from cave walls and dried-up ponds. The stone radical groups it with other mineral compounds: 砷 (arsenic), 硫 (sulphur), 硅 (silicon).
肖 (xiāo) supplies the sound, unchanged. No semantic contribution. Same seven-stroke phonetic drives a clean series: 消 (vanish), 销 (sell), 削 (slice), 宵 (night), 霄 (sky) - all xiāo or related readings.
