Unusual Fluorescence of a Color-Enhanced Amber Bracelet
A bracelet recently sent to Taiwan Union Lab of Gem Research (TULAB) for identification services contained amber that was pale brownish yellow, transparent, and slightly included (figure 1). Raman spectroscopy and microscopy confirmed the gems to be natural amber with an abundance of sun spangles, which were discoidal fractures caused by heat treatment.
Color enhancement of amber is generally not detectable unless the amber itself has cracks or pores extending to the surface during the baking process, in which case the color usually concentrates in these cracks and pores. Apart from that, amber that is color-enhanced by the baking process mostly shows inert or faint yellow fluorescence under long-wave ultraviolet light; however, such fluorescence may also occur in natural untreated amber.
Figure 2. Magnification revealed that the dark brown color was concentrated in the surface-reaching fractures or sun spangles, yet the sun spangles or fractures wrapped inside the amber were all yellow. Photomicrograph by Shu-Hong Lin.
Microscopic observation of the amber bracelet revealed that the dark brown color was concentrated in all the surface-reaching fractures or sun spangles, but the sun spangles wrapped inside all appeared in a lighter tone of yellow (figure 2). Thus, it was suspected that this amber bracelet had been baked to enhance color.In addition to the inclusion evidence, the amber bracelet’s long-wave ultraviolet fluorescence reaction unexpectedly presented a relatively bright blue along the ridgelines and inert to faint yellow on the rest of the stone (figure 3).
Figure 3. Under long-wave ultraviolet light, the amber bracelet presented a surprising bright blue fluorescence on the ridgelines and inert or faint yellow fluorescence on the other parts. Photo by Kai-Yun Huang.
This abnormal fluorescence strongly indicated that the amber had indeed been baked to enhance its color and that the ridgelines were worn due to grinding or wearing, which removed the brown surface layer.This case is worthy of attention because a series of similar items have subsequently been submitted for identification. Traditionally, the color enhancement of amber has been confirmed only by inclusion evidence; however, the abnormal fluorescence of this object offered supporting evidence.
