(Abstract of an article in Gem Information vol.42)
The blue colour of spinel is caused by iron and cobalt. But the peak positions of them are close and the absorption is overlapped in the spectrum. Thus is it difficult to separate its colouring agent from the position of its absorption with spectrometer. But the shape of the spectra, as the strength of some peaks, can give us an implication of which colouring agent is dominant. We separated them in to 4 categories (+1 synthetic category) as below. And compared with the data of LA-ICP-MS, they correlate as, Fe type, Co type, Low Fe + Co type and High Fe +Co type. And Low Fe + Co type was seen only in cobalt spinel from Vietnam.
Cobalt spinel from Vietnam has vivid and light colour, not dark as previous blue spinel from existing source like Sri Lanka or Tanzania. It is because they have Co content but low Fe content (or Fe does not contribute to its blue colour). And their inclusion is also very attractive. They have specific snow like needle inclusions. They are likely due to lamellar twinning and itself beautiful as photos.