**4. Discussion**

We have presented and analyzed all data from XRD, XRF imaging and micro-XANES of selected regions of the Dzi bead. We now attempt to understand what these results mean in connection with the questions we set out to explore: (1) what is this three-eye Dzi bead made of, (2) is it genuine or fake and (3) how was the crafting done?

We begin with the first question. Based on the XRD, XRF mapping and XANES, there is absolutely no doubt that this sample was made of SiO<sup>2</sup> (quartz) consistent with an agate origin. For the second question, since it is generally agreed by Dzi bead collectors and traders that Dzi beads are made of agate, our observations clearly show that it was made of SiO<sup>2</sup> (quartz), which eliminates all the fake possibilities illustrated in Figure 2c (plastic with metal core, polymer clay and painted aluminum).

The third question is perhaps the most interesting yet challenging and is most relevant to our analysis. Based on the descriptions of Bolin [1] and Ebbinghouse and Winsten [2], there are many ways to inspect Dzi beads and how they are crafted. We will focus on the surface-etched rings; the dark and light pattern; and how the XRF map, especially the Cu pattern we found in this analysis, may reveal how this three-eye Dzi bead was crafted.

From the optical image, we see that it showed well-defined light (white) and dark patterns, and the surfaces were covered with etched rings; these are the general characteristics of Dzi beads. Upon slicing the bead in half, we could see that the piece was translucent, and the white pattern did extend considerably into the bulk, as described by Ebbinghouse and Winsten [2]. None of the SEM, EDX, XRD and XRF mapping techniques showed any clear boundary between the light and the dark region, however. XRD of the powder showed the presence of graphite, while microdiffraction of the edge of the slice did not. It is unclear whether this is due to the sensitivity of the microbeam looking at a very small area or whether the powder was contaminated by carbon during the grinding process. This awaits future exploration. The presence of graphite may be related to light absorption in the dark region, however.

The most interesting finding was the hot spots of Cu on the circumference of the etched ring. In both Bolin's book [1] and the work of Ebbinghouse and Winsten [2], drilling with small bits and

the use of alkaline material and copper nitrate, Cu(NO3)2, were mentioned as agents put on the bead surface, followed by firing for whitening and darkening functions, respectively. The regular separation (~1 mm) of the Cu spots of ~20 µm in size could be speculated to be the tiny drill holes to facilitate diffusion of chemicals used in the pattern formation. The presence of CuO in all the Cu hot spots was likely the result of the following reaction upon firing:

$$\text{Cu(NO}\_3\text{)}\_2\text{ (s)} \rightarrow \text{CuO (s)} + \text{N}\_2\text{ (g)} + \text{3O}\_2\text{ (g)}$$

In addition to Cu, Ca and Fe were also found with both 12.0 keV (VESPERS) and 7.13 keV (SXRMB) excitation photon energy in the XRF map overlaying that of the Cu. K was also found in the tender X-ray map (not looked for in VESPERS); the origin of Ca is less clear, and Fe may have come from the tools used in crafting. It should be noted, however, that Fe is also commonly found in agate as a trace element, while Ca and K are not [18]. Compared to the Si map, which showed extruding pilings of SiO<sup>2</sup> on the circumference of the rings, the Cu spots appeared to be more like tiny pits. The Si and Cu map were correlated (Figure S4. It is also interesting to note that while we found CuO from the micro-XRF, it could not be detected in the XRD.

Finally, our preliminary finding was consistent with the descriptions noted in the literature [1,2]. Presumably, a piece of translucent agate was polished into shape and treated with alkaline solution. SiO<sup>2</sup> is slightly acidic, and a potash solution could do the etching (dissolving SiO2), and the light region could come from the increased porosity. The use of pyrolytic Cu(NO3)<sup>2</sup> and firing would produce CuO, which appears black. The origin of regular hot spots of CuO on the ring was possibly from drilling. The presence of graphite remains a mystery; repeated XRD measurements of various regions of the bead (not powder) revealed no crystalline carbon. The origin of carbon requires further investigation, although if plant sugar was used for darkening under heat and reed drill bits were used, it could be a source of carbon.
