Puy de Sancy is a very eroded lava dome rich in rocks with a porphyry structure with many crystals of sanidine. In 1900, the mineralogist, petrographer and geologist Alfred Lacroix sampled the rocks that constitute the dome and dubbed them ‘sancyites’. It was the exceptional abundance of sanidine, formed at high temperature and pressure, that led Alfred Lacroix to invent a new name for this original volcanic rock.