A rare type of emerald known as a trapiche emerald is occasionally found in the mines of Colombia. A trapiche emerald exhibits a “star” pattern; it has raylike spokes of dark carbon impurities that give the emerald a six-pointed radial pattern. Notice the hexagonal core, this shape is typical for the growth of emeralds.
As for the material forming the dividers, we read: “A TRAPICHE is the result of the growth of an Emerald Crystal with the darkened impurity of lutite. As the Crystal grows in its normal six-sided shape, the darker lutite is pushed to the center of the Crystal and then radiates out in the six directions of the corners of the Crystal. A 1970, analysis of MUZO´s trapiche emerald by Nassau and Jackson, found that the principal coloring agent was vanadium.
Trapiche Emerald’s are formed when black carbon impurities fill in at the emerald crystal junction which forms a radial pattern with a six-pointed star effect. In some trapiche emeralds, inclusions consisting of albite, quartz, and a carbonaceous material outline a hexagonal beryl core, and they extend from it in ‘spokes’ that divide the surrounding emerald material into six trapezoidal sectors. Often, the hexagonal beryl center is transparent and colorless, or it can be green.
This is not a case of asterism, but rather of crystal growth. So what is the mechanism that caused the creation of these beautiful and unique gems to form in this unique fashion? We have found the following: “The structure and mechanism of growth of these crystals, which have been found at the Pena Blanca mine near Muzo, Colombia, have been studied by Nassau and Jackson, who reported their results in a short paper in the Lapidary Journal, and in a more detailed report in the American Mineralogist.
Briefly, their proposed mechanism for the formation of trapiche emeralds is as follows:
First, the central, tapered core grows under hydrothermal conditions. Second, growth may slow or even stop for a while. Next, growth conditions change again, and both emerald and albite are formed. However, the hexagonal prism faces of the core crystal are able to maintain their uniform growth, producing pure emerald, while areas growing from the edges between prism faces are not, and are filled by albite. This results in six sectors of clear emerald, and six of predominantly albite and minor emerald. Thus, the central core and the six surrounding sectors of a trapiche emerald comprise a single, untwinned crystal.
It was long believed that these beautiful gems only came from Columbia, “These rare Emeralds are only found in Colombia occurring at Muzo and Penas Blancas mines. However we have found a reference to another location: “A large grayish green ‘trapiche’ beryl,”6 weighing 13.74 carats from Madagascar has been reported in Gems & Gemology. Information is very scarce on Trapiches from this area.
A quick word on treatments and enhancements is in order at this point: visitors to the actual mines recently report obviously treated material being sold as “natural”.
Treatment takes many forms, the most common is oil or epoxy impregnation of cracks, (this type of treatment is typical for most of the emeralds on the market.)
For a dissertation of the GIA approach for emerald grading see: Gems and Gemology, Winter 1999, “Classifying Emerald Clarity Enhancement at the GIA Gem Trade Laboratory” - by Shane F. McClure, Thomas M. Moses, Maha Tannous, and John I. Koivula, p.176-p.185.
In conclusion, if you are looking for a unique, beautiful, rare and wondrous gemstone the Trapiche Emerald truly fits the bill!