Gemmological and Spectral Characteristics of "Root Amber" from Sumatra

Root amber gets its name from the unique root-like appearance and is highly favored in the Chinese amber markets. However, natural root amber is mainly produced in Myanmar with limited output. Recently, a type of "root amber" with an appearance similar to root amber from Myanmar has emerged in the markets. This study systematically investigated "root amber" samples from Sumatra using multiple analytical methods, including microscopic observation, Fourier transform infrared spectroscopy, Raman spectroscopy, three-dimensional fluorescence spectroscopy, and scanning electron microscopy. The results showed that "root amber" samples from Sumatra exhibit significant differences from root amber from Myanmar in terms of colour, microstructure, spectral characteristics, and fluorescence properties. "Root amber" from Sumatra ranges in colour from milky white to brown, mainly divided into milky white transparent parts and brown opaque parts. Under long-wave ultraviolet light, it emits blue-white to blue-purple fluorescence, with the opaque areas showing stronger fluorescence than the transparent areas. It also has a distinct sticky texture and a resinous odor. Microscopic observation revealed reddish-brown spots of varying sizes in the transparent parts, while the opaque areas displayed a spongy, porous structure with numerous bubbles and flow lines formed by stress extrusion. SEM observation revealed structural differences between the transparent and opaque parts: The transparent parts are smooth and flat, while the opaque parts are porous, presumably due to bubbles and hollow tubes. FTIR analysis showed significant differences in mid-infrared characteristic absorption peaks between "root amber" from Sumatra and root amber from Myanmar. Specifically, "root amber" from Sumatra has unique alkane CH₂ antisymmetric stretching vibration peaks at 2 957 cm^-1 and 2 932 cm^-1, as well as a characteristic C-H out-of-plane bending vibration peak of C=C double bonds at 888 cm^-1, which is highly similar to copal from Borneo. According to experimental data and relevant literatures, it was concluded that the purported "root amber" from Sumatra is actually copal resin, exhibiting gemmological characteristics of typical type Ⅱ resin fossils. The low maturity of copal explains its sticky texture. The type Ⅱ resin fossil can easily lead to the misidentification of it as amber in maturity analysis based on the ratio of Raman spectral absorption peak intensities (I_{1 645 cm^－1}/I_{1 450 cm^-1}), which requires vigilance. Three-dimensional fluorescence spectroscopy analysis indicated that "root amber" from Sumatra emits light in the blue-purple region, differing from the fluorescence characteristics of amber from Myanmar. Its characteristic fluorescence peaks are concentrated around 445, 478 nm and 516 nm, with the emission range limited to the blue-purple region. The optimal excitation wavelength varies slightly (385-418 nm) depending on the oxidation degree of the samples. Compared with amber from Myanmar, it has fewer fluorescence emission centers, a narrower emission range, and no ultraviolet emission peaks. This study not only revealed the spectral and microscopic characteristics of "root amber" from Sumatra, but also provided a scientific basis for distinguishing with root amber from Myanmar.