Identification of Natural, Reconstructed, and Imitation Root Amber
Natural root amber is a variety of Burmese amber with a distinct appearance, good luster, and deep color (Y. Wang, Amber Gemology, China University of Geosciences Press, Beijing, 2018, p. 145). Root amber was named for its similarity in color and structure to tree roots. Root amber beads are popular in the Chinese market, and the Guangzhou Gemstone Testing Center receives many of them for identification. Recently we have noticed three different types submitted for identification (figures 1 and 2): natural root amber, reconstructed root amber (assembled from either pressed powders or small fragments), and plastic imitations.
Specific gravity (SG) was measured for three natural Burmese root amber samples (N-1 through N-3), three samples reconstructed from pressed powders (M-1 through M-3), three samples reconstructed from pressed fragments (M-4 through M-6), and three plastic imitations (P-1 through P-3). The SG of most varieties of regular Burmese amber samples is within 1.020–1.050, except for root amber and reddish amber (1.060–1.110). Overall, the natural root amber had higher SG (1.044–1.107) than that of regular Burmese amber and much lower SG than that of the reconstructed root amber from pressed amber powders (1.133–1.137) and the plastic imitations (1.161–1.167). The root amber reconstructed from pressed fragments, meanwhile, had an SG of 1.075–1.077. The variations in SG of the reconstructed amber were due to differences in internal structure and/or gas porosity.
Most importantly, the internal features of the samples were quite different. There were some cellular or porous structures in the natural Burmese root amber (figure 3A). The typical red-brown dots of Burmese amber could be detected in both transparent and opaque zones (figure 3B). The flow-like pattern of natural Burmese root amber can be lamellar, annular, or flocculent, and there are natural transitions between varying colors (figure 3C). The internal features of the root amber reconstructed from pressed powders displayed an unnatural flow-like pattern similar to the veins of a leaf (figure 3, D–E). Under higher magnification, these samples displayed a granular texture (a sugar- or sand-like pattern with apparent broken particles), confirming that small amber micro-particles had been pressed (figure 3F). There were very clear and rigid borders in the root amber reconstructed from fragments (figure 3, G and H). Some granular texture could still be found in these samples (figure 3I). The flow-like pattern of plastic imitation is mostly ribbon-, string-, or ring-like, with a sense of separation between the brown matrix and the obvious boundaries (figure 3, J–L).
Using Fourier-transform infrared (FTIR) spectroscopy, the plastic imitations were quickly identified in the mid-infrared range (3500–500 cm–1). The extremely strong peak at 1731 cm–1, three peaks of increasing intensity (1240, 1192, and 1148 cm–1), and other peaks suggest that the material is an amino resin (figure 4, blue line). The vibration peaks of functional groups of natural root amber mainly appear at 2928, 2860, 1720, 1458, 1375, 1227, 1141, 1028, and 974 cm–1, consistent with the main FTIR peaks of Burmese amber (figure 4, green line). The obvious “Baltic shoulder” (Y. Wang et al., “Characteristics of hydrothermally treated beeswax amber,” Fall 2019 G&G, pp. 370–387), composed of a broad and weak absorption shoulder at 1265 cm–1 and a sharp peak at 1158 cm–1 as well as an absorption peak near 887 cm–1, appeared in two reconstructed root amber specimens (figure 4, pink and purple lines). It can be inferred that these two reconstructed samples were pressed from rough Baltic amber.
