Origin

This character combines stone (石) on the left, which classifies it as a mineral substance, with a sound hint (肖) on the right that suggests how to pronounce it. Saltpeter and other nitrates are crystalline mineral salts that form in or are extracted from stone and soil, so pairing the stone radical with a character indicating the pronunciation makes sense for naming this chemical substance. The left component tells you the character refers to something mineral-like, while the right component is just there to guide your pronunciation, not to add meaning.