The Spanish word 'piedra' meaning 'stone' comes from the Latin word 'petra', which also meant 'rock' or 'stone'. The Latin word itself was borrowed from Ancient Greek 'πέτρα' (petra), meaning 'rock' or 'stone'. The evolution from Latin 'petra' to Spanish 'piedra' shows a common sound change in Spanish where the 'e' becomes a diphthong 'ie' in stressed syllables.

Some related Spanish words include 'pedregal' (rocky ground), 'pedrero' (stonemason), and 'pedrería' (precious stones). These words maintain the 'pedr-' root from the same Latin origin. The word 'Pedro', a common Spanish name, is also related, as it comes from the Latin name 'Petrus' meaning 'rock' or 'stone'.

Several English words share this Greek-Latin origin. 'Petroleum' literally means 'rock oil' (from petra + oleum). 'Petrify' means 'to turn to stone'. The name 'Peter' also comes from the same root, sharing its origin with Spanish 'Pedro'. The scientific term 'petrology' (the study of rocks) also contains this ancient root.