|Trapiche emerald from Muzo Mine, Colombia.|
Credit: Left Photo: Luciana Barbosa, Right Photo: © Martin Slama
Trapiche emerald is a rare and highly valuable variety of emerald gemstone that features a unique star-like pattern (like wheel spokes). This beautiful gem variety is highly revered by the serious gem collectors and high-end jewelry designers for making expensive jewelry pieces.
The radial pattern exhibits considerable variance, and often includes a hexagonal structure at the core. There is not yet consensus about the mechanism by which the pattern forms, or the conditions required for it. Various models have been proposed.
Despite its starlike appearance, this unique “spoked” pattern isn’t a case of asterism (the “star stone” effect). However, trapiche emeralds may reveal chatoyancy, a “cat’s eye” effect. Parallel growth tube inclusions can create a cat’s eye in the “pie-shaped” sections as well as, rarely, along the length of whole cabbed trapiche emeralds. Expert lapidaries can orient and cut these stones to bring out this effect.
When emerald miners first discovered these patterned gems, they called them trapiche, the Spanish word for the multi-spoked cogwheels rotated by oxen tied to a yoke to grind cane sugar. They were first described in 1879 by French mineralogist Emile Bertrand at a meeting of the Société Géologique de France.
Trapiche emeralds are found in the black shale host rocks of just a few Colombian mines in the western belt of the Eastern Cordillera Basin. Their appearance consists of a central core, six arms and black shale dendrites – a crystalline, branching tree-like structure that forms between the arms and around the core. They are visible when the crystals are viewed perpendicular to the c-axis and are often cut as cabochons to accentuate the six partitions.
|Very rare crystal Trapiche Emerald from Muso Mine, Colombia. Photo: Cricksgemstone|
During the formation of an emerald crystal, black carbon impurities may enter the gemstone mix. Because of emerald’s hexagonal crystal structure, these impurities may fill in at the crystal junctions, forming a six-point radial pattern.
In some trapiche emeralds, inclusions of albite, quartz, carbonaceous materials, or lutite may outline the hexagonal emerald core. From there, they extend in spokes that divide the surrounding emerald material into six trapezoidal sectors.
The extraordinarily rare trapiche emeralds are primarily found in the Muzo, Coscuez and Peñas Blancas mines of Colombia. The trapiche pattern is not an asterism, which is a six-rayed star pattern sometimes seen in cabochon-cut rubies, sapphires and other gemstones.
See also: How Does a Trapiche Emerald Form?