corundum, sapphire, treatments.6, surface repair, glass infilling, Nigeria.12, Montana.12, gems

A glass filling in a faceted sapphire is investigated by electron microprobe analysis and compared with previously analysed glasses in ruby to assess whether it formed naturally or through treatment. The principal specimen is a 1.28 ct round mixed-cut sapphire with a known history extending back to 1979. A surface-reaching glassy area near the culet measures less than 1 mm across and contains numerous bubbles. Such fillings can be detected by careful examination in reflected light or by immersion, which helps reveal small bubble-bearing areas. Ten analyses give a mean composition of 57.5 wt% silica, 29.4 wt% alumina and 6.7 wt% sodium oxide, with smaller amounts of iron, calcium, potassium and magnesium oxides. Although individual analyses vary, the composition broadly resembles that of a previously studied ruby filling, with differences particularly in sodium, potassium and calcium contents. Three possible origins are considered: natural volcanic glass adhering to corundum, artificial glass introduced during treatment, and glass produced by melting pre-existing mineral inclusions. A volcanic origin is geologically conceivable because sapphires occur in basaltic deposits and may undergo corrosion during transport in magma. However, the filling differs substantially from typical alkali basalt and from reported residual volcanic glasses, especially in its high alumina content. Available experimental data do not permit every possible natural glass composition to be excluded. Treatment materials, including coatings, pastes and crucibles, could supply the elements needed to form artificial glass. Element maps show an essentially homogeneous filling and a sharp boundary against the sapphire, without an aluminium gradient supporting dissolution of the host as the source of alumina. The high melting temperature and viscosity expected for such glass nevertheless make it an unusual choice for intentional filling. No known mineral closely matches the filling’s sodium–aluminium–silicon proportions. Heating experiments on jadeite at 1200–1600 °C show insufficient sodium loss to explain the difference by volatilisation during melting. The authors conclude that the glass in the examined sapphire is artificial. More generally, glass fillings in corundum have varied compositions: some may derive from melted natural inclusions, whereas others are more plausibly associated with artificial treatment.