FOR over 1,000 years, the emerald deposits of Colombia have been the principal source of the world’s largest and finest gem-quality emeralds-a variety of beryl containing chromium and vanadium(1). Whereas most emerald deposits are found in association with igneous host rocks(1), the Colombian deposits occur in organic-rich black shales, and their origin in the absence of any evidence of igneous activity has been a persistent enigma. Here we present evidence from the Muzo mine (located about 100 km from Bogota) that hydrothermal brines transported evaporitic sulphate to structurally favourable sites, where it was thermochemically reduced. We suggest that the sulphur generated by this process reacted with organic matter in the shales to release trapped chromium, vanadium and beryllium, which in turn enabled emerald formation.