inclusions, crystal angles, gems

In a letter published under “Flashback” in Gems & Gemology, Edward Gübelin described how he identified a quartz crystal enclosed in a Colombian emerald by measuring angles from photomicrographs. He was prompted to write by Figure 11 on page 213 of the Fall 1976 issue, where the GIA laboratory notes showed a quartz inclusion in an emerald from Colombia. The image immediately reminded him of an inclusion he had examined in his own testing laboratory, and he believed the published photograph showed the same emerald. During his examination, Gübelin had taken several photomicrographs from different angles because the enclosed crystal had a sharply defined form. He expected that its well-developed point would allow him to measure angles that could establish the mineral’s identity. He made sketches of the crystal from the photographs and took his measurements from those drawings. The accompanying diagram shows two views of the six-sided termination and gives angles of 76°30′ perpendicular to the a-axis and 84°30′ parallel to it. Gübelin reported that these values were typical of quartz. He also observed that the positive and negative rhombohedra forming the point were equally developed and remarked that a freely grown quartz crystal could have a blunter termination. The agreement between the measured angles and those characteristic of quartz led him to conclude that the crystal enclosed in the emerald was quartz on morphological grounds. The editor presented the letter as an example of how a reader’s observations could supplement a published laboratory report. The original photograph documented the appearance of the inclusion, while Gübelin’s sketches and measurements provided the geometrical basis for his identification. The case illustrates how photographs taken from different orientations can record enough of a well-formed inclusion’s geometry to identify the mineral while it remains enclosed in its host. Here, the six-sided point supplied two measurable angles that could be compared with those of quartz.