gems, beryl, emerald, Colombia, Chivor, Gachalá, Muzo, Cosquez, Peñas Blancas, beryl, emerald cat’s eye, beryl, emerald fossils, trapiche beryl, emerald

Recently the Matecaiia tunnel, located in the Gachalâ emerald mining area, has reached a level producing some gastropods which shell has been replaced by emerald. In this study involving geology, mineralogy, gemmology and palaeontology, the Gachalá mining area and the history of ils discovery are described. Twenty-four samples representative of the embedding rocks, three facetted stones and twenty fossils are studied; the genesis of the events responsible for emerald crystallisation is explained in detail, the fossils are identified and the phenomena leading to the substitution of the shell material is presented.