Growth pattern observed in a trapiche-type emerald from Swat, Pakistan. The slice shows two colorless cores and two intermediate lighter green zones surrounded by a more intense green rim. Viewed perpendicular to the c-axis, in air (left) and immersion (right); diameter of the sample from 4.6 to 4.8 mm. Photos by Karl Schmetzer.
Numerous gem materials have been designated “trapiche” in the trade and in various publications. They generally describe a pattern consisting of a fixed non-transparent star in a transparent matrix, with the arms of the six-rayed star radiating from a central point or from a central core to the rim of the crystal. Only a limited number of mineral species show a clear trapiche pattern within crystal slices (i.e., a pattern with a clear separation of the crystal into distinct growth sectors). The boundaries of these growth sectors, running from the center to the edges between external prism or pyramidal faces, are normally sharp and contain mineral or fluid inclusions, separating different parts within the rim. These sector boundaries are also described as the arms of the fixed six-rayed star. The most prominent gem minerals showing a distinct trapiche pattern are Colombian emeralds and rubies from Myanmar.
Another group of gem materials is characterized by several areas of increased transparency and translucency that surround a central point or central core according to the symmetry of the host. The two types of areas are characterized by different concentrations of inclusions, which are trapped within specific parts of symmetry-equivalent growth sectors of the host. The arms of the fixed star are perpendicular to the external crystal faces. The most prominent gem materials of this second group are blue basaltic sapphires from different sources, but similar patterns also have been described for other gem varieties such as aquamarine from Namibia. This second group of crystals is designated “trapiche-type,” and the emeralds from Pakistan described in this entry belong to this group of gem materials.
Trapiche-type emeralds originating from the Swat mining region in Pakistan have recently been described by Y. Gao et al. (Fall 2019 Gem News International, pp. 441–442) and H. Guo et al. (“Inclusion and trace element characteristics of emeralds from Swat Valley, Pakistan,” pp. 336–355 of this issue). The samples showed an interesting growth pattern and color zoning with a colorless core, a lighter green intermediate zone, and a more intense green rim. The inclusions forming the trapiche-type pattern were restricted to the lighter green intermediate growth zone.
Recently, the author received six slices of similar material, reportedly from Swat, from the gem collector S. Hanken of Waldkraiburg, Germany; the samples had been purchased in 2020 from the U.S. gem trade. One of these samples, which were all cut perpendicular to the c-axis of the emerald crystals, showed an interesting growth pattern. The emerald slice measured from 4.6 to 4.8 mm (distances between different prism faces) with a thickness of 1.9 mm (see above). The sample revealed two colorless beryl cores of almost equal size. One of these colorless cores was surrounded by a large light green intermediate zone, which also contained the inclusions forming the trapiche-type pattern. The second core was surrounded by a lighter green intermediate zone, which was much smaller. Encompassing these two intermediate zones were intense green hexagonal growth boundaries with growth planes parallel to the external prism faces.
This intense green growth boundary surrounds both light green intermediate growth zones. Subsequent to this intense green boundary, an intense green outer rim without trapiche-type inclusions but with additional growth planes parallel to the prism faces was present.
As already described by Gao et al., the emerald was grown in three distinct steps with a colorless beryl core, a light green intermediate zone, and an intense green rim. All growth steps were separated by sharp boundaries, with a thin growth layer between the intermediate zone and the rim, which showed the most intense green coloration observed in the sample. The emerald formed with two cores, with a subsequent growth step, in which both parts were still separated from each other. Only in the last growth step, in which the intense green rim was formed, were the two parts of the final emerald crystal in contact with each other.
Source: gia.edu Karl Schmetzer