Gem News International Gems & Gemology, Fall 2019, Vol. 55, No. 3

Trapiche Emerald from Swat Valley, Pakistan

Yujie Gao, Xueying Sun, and Mengjie Shan

Download PDF
Six emerald wafers displayed a trapiche-like pattern.
 
 
Figure 1. Six emerald wafers reportedly from Swat Valley, Pakistan, showing a trapiche-like pattern. They range from 0.83 to 0.38 ct from left to right, with a thickness of about 1.51 to 2.08 mm. Photo by Kaiyin Deng.

Trapiche emeralds are usually found in Colombia. Recently the authors received six emeralds reportedly from Swat Valley, Pakistan, polished as double-sided wafers, retaining their original hexagonal crystal habit and exhibiting a trapiche-like pattern (figure 1). These samples weighed from 0.38 to 0.83 ct, with a refractive index of 1.588–1.599 and a birefringence of 0.009–0.011.

Four parts generally make up the trapiche emeralds from Pakistan.
 
 
 
Figure 2. The trapiche emeralds from Pakistan were mainly composed of four parts: a green rim, a light green area, six arms, and a nearly colorless core as illustrated in figure A. These arms appear dark under transmitted light (B) and show bright metallic luster under reflected light (C); field of view 8.48 mm. Illustration and photos by Yujie Gao.

Generally, these trapiche emeralds were composed of four parts from rim to core: a green rim, a light green area, six arms, and a colorless core, as illustrated in figure 2A. The rims had a highly saturated green color, and most were relatively clean except for several fractures and tiny fluid inclusions. The rims ranged from approximately 1 to 2 mm wide. Although the boundary between the green rim and the light area was not very sharp, a hexagonal boundary was visible. Six black arms spread in a hexagonal symmetric pattern, with each arm perpendicular to the hexagonal side. The colorless core usually had a hexagonal shape.

Raman analysis identified magnetite inclusions in the emerald host.
 
 
 
Figure 3. Raman analysis identified these inclusions as magnetite. Peaks at 662 and 545 cm–1 agree with the two main peaks of magnetite, according to the RRUFF database, while peaks at 683 and 400 cm may be assigned to the emerald host.

Microscopic observation revealed that every arm contained many minute black platy inclusions. These small particles appeared dark under transmitted light (figure 2B), while they showed bright metallic luster under reflected light (figure 2C). Micro-Raman analyses (figure 3) identified these inclusions as magnetite. Peaks at 662 and 545 cm–1 were consistent with two main peaks of magnetite, according to the RRUFF online database (rruff.info), while peaks at 683 and 400 cm–1 may be assigned to the emerald host. Chemical analysis by energy-dispersive X-ray fluorescence (EDXRF) on the sample shown in figure 2 revealed an iron content of 20,860 ppm (n = 3) in the light green area and 17,000 ppm (n = 3) in the rim. The difference in iron content may be due to the inclusions, since magnetite (Fe3O4) is mainly composed of iron and oxygen.

To our knowledge, there have not been many reports of trapiche emerald from localities other than Colombia. The trapiche pattern caused by platy magnetite inclusions could help advance our understanding of trapiche.

Yujie Gao, Xueying Sun (shirley.sun@guildgemlab.com), and Mengjie Shan are with Guild Gem Laboratories in Shenzhen, China.