Colombian Trapiche Emeralds: Recent Advances in Understanding Their Formation

• Published in: Gems & gemology 2015-11, Vol.51 (3), p.222-259
• Main Authors: Pignatelli, Isabella, Guiliani, Gaston, Ohnenstetter, Daniel, Agrosi, Giovanna, Mathieu, Sandrine, Morlot, Christophe, Branquet, Yannick
• Format: Article
• Language: English
• Subjects: Earth Sciences; Mineralogy; Sciences of the Universe
• ISSN: 0016-626X; 2376-4473
• DOI: 10.5741/GEMS.51.3.222

Colombia is the traditional source of the world’s finest emeralds, including the famed trapiche crystals, with their distinctive texture resembling a wheel with six spokes. This gemological curiosity, found exclusively in the black shales of the country’s western emerald zone, is linked to the peculiar structural geology of the deposits. The study presents a review and update on Colombian trapiche emeralds, followed by a three-dimensional examination of the crystals combined with spectroscopic and chemical analyses. The proposed formation model incorporates the structural geology of the deposits with the formation of trapiche and non-trapiche emeralds. The fluid accumulation at the faults’ tip in the black shales leads to maximum fluid overpressure and sudden decompression and formation of the emerald-bearing vein system. The authors show that trapiche emerald growth starts at the beginning of the decompression that is responsible for local supersaturation of the fluid. The hydrothermal fluid comes in contact with the black shale matrix, favoring the formation of emerald seed crystals. During the growth of these seeds, textural sector zoning occurs, sometimes associated with chemical sector zoning, along with displacement of the matrix. Displacement growth occurs because the emeralds continue their growth, pushing the matrix material away from the growing faces. An overgrowth, generally of gem quality, can form after decompression, surrounding the core, the arms, and the dendrites, restoring the emeralds’ euhedral habit.