Characterization of heat treated geuda gemstones for different furnace conditions using FTIR, XRD and UV–Visible spectroscopy methods

Abstract

Geuda is a variety of corundum gems commonly found in Sri Lanka and Madagascar. Geuda lack colour and clarity compared to other sapphires yet has the potential to convert into quality blue sapphires by heat treatment at high temperatures. Gas-fired furnaces (GF) are commonly employed in the gem trade to heat treat Geuda. Recently introduced electric furnace (EF) has also shown potential in heat-treating Geuda with variable success though the conditions inside the EF differ from GF. This study investigated the colour enhancement potential of geuda after a two-step heat treatment under different temperatures and soaking periods where GF and EF furnaces were used in combination. Resulting gemstones were characterized using FTIR, XRD and UV–Visible spectroscopy methods. There was a significant peak height drop at 3309 cm−1 in the FTIR spectrum after heat treatment, that corresponded to O–H stretching mode of water inside the stones. UV–Visible spectroscopy showed a peak development after 550 nm due to the formation of the blue colour [FeTi]6+ complex inside heat-treated stones. XRD analysis has shown that the interplanar distance of the lattice structure of heat-treated geuda stones has changed due to the random movement of atoms inside the lattice. Two-step heat treatment, GF at 1700 °C with one month soaking period followed by EF at 1750 °C with 30 min of soaking under reducing atmosphere, has produced the desired blue colour in Geuda stones. This study further revealed that heat-treated geuda stones could effectively be identified from natural sapphires using FTIR, XRD and UV–Visible spectroscopy.