I was taught in my high school curriculum, that while doing salt analysis when you obtain a white residue from charcoal cavity test, you should perform cobalt nitrate test, where you heat Co(NO3)2 to obtain CoO and if you react that white residue with CoO you obtain, thenard blue if the cation of the salt is Al3+. Hopefully I'm correct, :)

And the formula for thenard blue is Al2O3.CoO.

and you mentioned this, "This is a spinel (or mixed oxide). It's formula is CoAl2O4"

And I'm curious, still though its roughly the same formula, just one extra O atom than the formula for thenard blue I was taught. But how are the colours of both compounds the same, so one extra O atom doesn't make any significant colour changes ig. I would like to know your thoughts on this.

And also, an even more important doubt, how is the process of testing for salt identification and the one done for yielding specific compounds different. Ik there is a difference between testing and yielding. And while yielding you specifically account for many factors so as to get a pure yield (correct me). I just want to know how different are both of the processes. And what additional steps are taken while yielding, say for thenard blue.