The geology of the Muzo is similar to that of Chivor, but the emerald-hearing veins, containing calcite, quartz, dolomtite and pyrite, run through a black carbonaceous limestone and shale which form the country rock. Crystals are hexagonal prisms and often of a magnificent colour with a mean SG of 2.70 and mean RI for the ordinary and extraordinary rays respectively of 1.58 and 1.57 with DR 0.005–0.006. Three-phase inclusions are characteristic, containing a salt crystal, liquid (water or brine) and gas bubble (carbon dioxide). Calcite crystals have not been recorded as inclusions in Muzo emeralds. Brownish crystals of the cerium carbonate, parisite, are, however, characteristic.
The name trapiche emerald is given to crystals with a cog-wheel-like core and spokes of fine green emerald which occur both at Muzo and Chivor, the name deriving from a Spanish word for cane-crushing gear. Cores in Muzo trapiches tend to be darker than in Chivor specimens. Buena Vista emeralds are found in an area with similar geology and mineralogy to that of Chivor; the group of mines is in the municipality of Ubala. There are other emerald-producing sites in Colombia. The Gachala area is south-east of Muzo and closer to Bogotà; in the 1970s emeralds of attractive colour were on the market in Germany for a short time. The colour is attractive with noticeable banding. The geology and mineralogy are similar to that of Chivor. Large crystals of the finest emerald are almost certain to have come from Colombian sources; in Smaragde unter dem Urwelt (1941; Sinkankas #3456) Fritz Klein gives full details of the recovery of a large emerald crystal ‘Patrizius’ (sometimes ‘Patricia’), which later went to the American Museum of Natural History, New York. A picaresque account of emerald mining in Colombia can be found in Peter Rainier’s Green Fire (1942; Sinkankas #5313 and several later editions). The area described is Chivor and neighbouring Somondoco.