Purple substance in hollow tubes to mimic unheated Paraiba tourmaline
By Dr. Michael S. Krzemnicki, first published in Facette 30 (March 2026)
Recently, the SSEF received a 14-carat, light blue, copper-bearing tourmaline for testing (Figure 1). Based on trace element concentrations, its Mozambique origin was readily established. Microscopic and spectroscopic features, such as the absence of a manganese-related absorption band in the UV-Vis spectrum and numerous discoid fissures extending from fluid inclusions, clearly indicated that this tourmaline had undergone heat treatment to enhance its colour.
The most interesting aspect, however, was the microscopic observation of the hollow channels, which were partly stuffed (i.e. ‘closed’) with a purplish-blue substance where these channels reached the gemstone’s surface (Figure 2). While it is possible that this is pure coincidence, we consider it more likely to be a deliberate attempt to mislead gemmologists into thinking the stone is unheated.
It is well known that natural irradiation due to radioactive substances in open hollow channels in Paraiba tourmalines can create a purple halo around the channels over geological time periods (see also Facette, 29, 2024, p. 14). In fact, the presence of such a halo is considered evidence that no heat treatment was applied to the stone. Heating would cause this purple halo to disappear.
However, as mentioned, this natural purple halo (created by activating a local manganese-related colour centre) is found around the hollow channel and not inside it. A classic example of such an unheated case is a 55-carat copper-bearing tourmaline from Mozambique, which reveals a purple halo around a tiny hollow channel and orange-brown iron hydroxide (goethite) within the channels (Figure 3a,b). This is only stable up to around 350 °C, which is below the heating temperatures (400–450 °C) commonly applied to Paraiba tourmalines.
