Micro-World Gems & Gemology, Winter 2020, Vol. 56, No. 4

Unmasking Emerald Filler

Irregularly shaped cavities in emerald.
Figure 1. An emerald with irregularly shaped cavities, shown in darkfield illumination. Photomicrograph by E. Billie Hughes; field of view 4 mm.

Of the treatments we see in the laboratory, fissure filling has become one of the most ubiquitous. We have observed this treatment in a variety of stones, including emerald, ruby, sapphire, spinel, tourmaline, tanzanite, and more. The filling of fractures minimizes their appearance, making the gems appear cleaner. Fissure filling can be detected with a variety of methods. One way is to examine the infrared spectrum, where some fillers display distinctive peaks. Another is the hot point method, where oils can be observed leaking out in droplets. Some fillers are unmasked with simple observation in the microscope, because they display flashes of color or because they include visible dyes.

In addition to these methods, another tool in our arsenal is the long-wave ultraviolet flashlight, which can be used in conjunction with the microscope. Because some fillers display a chalky fluorescence when illuminated with long-wave UV light, shining a long-wave UV flashlight at the specimen is a simple technique that helps us to not only to detect the filler, but also to see its exact location in the stone. This helps the gemologist gauge the extent of filling and its impact on the stone’s overall appearance.

The cavity fluoresces chalky blue to long-wave illumination.
Figure 2. When the same inclusion scene is lit with a long-wave ultraviolet flashlight, it becomes evident that the cavity contains a filler that fluoresces a chalky blue color in long-wave illumination. The irregular shapes can also be seen to contain rounded gas bubbles. Photomicrograph by E. Billie Hughes; field of view 4 mm.

Figure 1 offers a great example of this. This inclusion scene in emerald shows an irregular cavity. When illuminated with a long-wave UV flashlight (figure 2), it is immediately evident that the cavity is filled with a substance that displays a chalky blue fluorescence. Observation under long-wave UV light also makes it easier to observe gas bubbles in the filled areas.

E. Billie Hughes is a gemologist and co-founder of Lotus Gemology in Bangkok.