Trapiche Emerald from Colombia
Colombian trapiche emeralds, in general, consist of a transparent or nontransparent tapered core and six transparent prismatic growth sectors, which are separated by nontransparent boundaries. These boundaries form the arms of a six-rayed fixed star and radiate from a central point or a central area within the conical core. The tapered core is also designated a basal or pinacoidal growth sector. The boundaries consist of emerald, various minerals (e.g., calcite and albite), and trapped fluid phases.
In the gem trade, trapiche emeralds are mostly cut as slices or cabochons with an orientation perpendicular to the sixfold axis of the emerald to show the hexagonal growth pattern of the gemstone and especially to display the six-rayed fixed star. The conical shape of the core is derived from the different diameters of the core on both sides of such a cabochon. Slices of trapiche emeralds with an orientation parallel to the c-axis clearly show a tapered core, representing the basal growth sector, and a rim on both sides of the core formed by two prismatic growth sectors. However, because rough trapiche emerald specimens are mostly broken on both ends, the complete trapiche pattern parallel to the c-axis is not observed, and even textbooks dealing with Colombian emeralds show only sketches or photos of crystal fragments.
Recently, a complete outline of a trapiche emerald pattern consisting of two pinacoidal (basal) and six prismatic growth sectors was presented by I. Pignatelli et al. (“Colombian trapiche emeralds: Recent advances in understanding their formation,” Fall 2015 G&G, pp. 222–259), but only fragments of trapiche crystals sliced parallel to the c-axis were available for that study. Consequently, the present author took the opportunity to examine the pattern in a transparent sample which, on a first view, seemed to be a complete or almost complete trapiche emerald crystal. The emerald was submitted by collector Georg Sellmaier (Kranzberg, Germany), who purchased the sample about 10 years ago from a local dealer while traveling in the Muzo area of Colombia.
The 3.49 ct emerald crystal (figure 1A) shows a prismatic habit with six elongated prism faces m (1010) and is terminated on one end by a basal pinacoid c (0001) and six small hexagonal dipyramids p (1012). Most likely, at the second termination of the crystal a facet approximately in the direction of a basal pinacoid was cut because the sample was slightly broken or did not show a plane face. The length of the crystal is 15.4 mm, while the diameter between the prism faces is 4.6 mm and the diameter between the crystal edges is 5.2 mm. Its pleochroism is blue-green parallel to the c-axis and yellow-green perpendicular to the c-axis.
In an orientation where the crystal is resting on one of the six prism faces (i.e., in a view perpendicular to this face), three different prisms are seen. In a view through the central prism, a cross in the form of an X was observed, with a pattern described as part of such a cross seen also in views through the two other adjacent prism faces (again, see figure 1A). A schematic drawing of this situation is presented in figure 1B. In the immersion microscope, upon rotation of the emerald crystal along its hexagonal axis, in each view parallel to two of the six prism faces (i.e., rotated by 30° versus the view perpendicular to a prism as described above and depicted in figure 1), a pattern consisting of a sharp cross is seen, with a vertical line dividing the cross into two parts (figure 2, A and B). The schematic drawing in figure 2C reflects this situation. In a view oblique to the c-axis, a tapered cone formed by six plane boundaries is seen at both ends of the crystal (figure 3). In a view parallel to the c-axis, due to the length of the crystal and the various layers covered with inclusions present throughout the crystal, the sample is only translucent, but a pattern of a six-rayed fixed star is observed.
It is evident that the various lines forming crosses in the form of an X are the traces of plane boundaries between the two pinacoidal and the six prismatic growth sectors. The vertical lines that subdivide these crosses in the second orientation described here represent boundaries between the six prismatic growth sectors (see figures 1B, 2C, and 4A). The cone-shaped outline of each pinacoidal growth sector is formed by boundaries between the tapered core and six prismatic growth sectors (figure 4B). The different patterns depend only upon the orientation of the crystal in the direction of view and the orientation of the various boundaries between the six prismatic and two basal growth sectors within the emerald.
The crystal described represents an almost complete emerald with prismatic habit and trapiche pattern. In the present case, the trapiche pattern is similar to an hourglass structure. However, the trapiche emerald shows numerous inclusions that are trapped at the boundaries between different growth sectors, forming thin layers. As described in the literature, these layers between the individual emerald growth sectors consist of a mixture of emerald with other minerals and fluid phases trapped in cavities. In contrast to most trapiche emeralds from Colombia, the layers in the emerald described in this paper are thinner and translucent, which allows an observation of the complete three-dimensional trapiche pattern, especially in views perpendicular to the c-axis. This observation is consistent with the conclusion made by several authors about the cone-shaped cores of trapiche emeralds and confirms the three-dimensional model of the trapiche pattern presented by Pignatelli et al. (2015).
