gems, spinel, corundum, spinel imitation, blue spinel

The colour of most blue spinel is attributed to the presence of Fe, resulting in a slightly greyish blue hue. However, a trace amount of Co (in the absence of elevated Fe) can cause vibrant blue colouration in both natural and synthetic spinel (Schollenbruch et al. 2021). Co-blue spinel is among the rarest varieties of this Mg-Al oxide. In recent years, a new coating treatment has emerged on the market, mimicking Co-blue spinel by depositing a Co-rich film on the surface of synthetic sapphire, reportedly using a high-temperature flux method. Cabochons, beads and faceted gems have been treated by this technique, but faceted samples are less commonly encountered on the market since they tend to have lower lustre than the other types. Two samples of coated synthetic sapphire beads (Figure 33), each weighing 18.1 ct, were selected for characterisation in order to distinguish them from natural untreated Co-blue spinel. Permission was granted to slice one piece to facili- tate examination.