Natural Emerald : Unusual Curved Zoning Observed by GIA

Unusual Curved Color Zoning Observed in a Natural Emerald by GIA. Photo by Sasithorn Engniwat.

I first saw this report by GIA gemologists, Vincent, Nathan & Supharart about a month back. Such curved zoning is typically associated with synthetic corundum & synthetic spinel. However, for the first time (at least the first time reported) such an unusual color zoning has been observed in a Natural Emerald from Pakistan.

The Emerald weighs 1.17 carats & the origin has been determined to be from the Swat Valley of Pakistan.

Quoting the GIA report,

The gemological properties showed an RI of 1.589–1.597 and a visible spectrum consistent with emerald. The stone showed no reaction to a standard 4-watt gemological UV light source. Microscopic examination revealed numerous inclusions of pyrite, carbonates, and reflective thin films oriented perpendicular to the optic axis (parallel to the length of the stone), a typical assemblage of a natural emerald. The most unusual observation about this stone was a prominent green “S” shape color zone that ran down its length (figure 1). Emerald’s hexagonal structure would ordinarily lead one to expect planar or angular color zoning related to the crystal morphology. The presence of curving zones in a faceted emerald was an anomaly that needed further examination.
One possible explanation for the unusual zoning is that compressional geological forces caused shearing along the emerald crystal’s basal plane. This was supported by the appearance of pyrite grains, which looked as if single, brittle pyrite crystals had been sheared and slightly displaced parallel to the basal plane of the beryl host
This apparent lateral shearing movement throughout the emerald crystal could have caused the pyrite grains to separate into aligned tabular fragments offset from each other and the beryl host. Also observed between the pyrite grains were planar optical irregularities. These planes suggested localized shear zones that would have significantly higher defect concentration, giving rise to the optical nonconformity consistent with their location between the tabular pyrite inclusions. Under cross-polarized illumination, we observed dense clusters of birefringent crystals that were essentially invisible under non-polarized lighting. The low relief of these dense inclusions suggested they were beryl inclusions within the beryl host, possibly a result of partial recrystallization caused by dynamic environ­mental conditions.
The discovery of such gems demonstrates that there are complex geological conditions we do not yet understand which, when observed, invite further gemological exploration.

However the actual reasons for this unusual zoning could not be determined due to limitations of non-destructive gemological sciences & methodologies.

The original report can be found at GIA's report on the Unusual Curved Color Zoning in a Natural Emerald.