Vivid blue, oh where are you? Electric blue Spinel gemstones

[Barrett]

I just purchased a lot of cut stones from a new guy I hadn't messed with before.
Being my first dealings with this person, I did like I normally do, and bought a fraction of the parcel, with the understanding I would get the whole batch, once the first stones arrive and check out properly. I have 3 of these Vietnamese Cobalt spinels coming for this first deal we are doing together. I really need to get the 3 stones tested(at least one) before I drop a bunch more cash on the rest of the stones. I am nervous about cobalt doped syn. spinel being switched out for natural spinel. I recall syn. spinel being used as an imitation for tanzanite.

Any ideas as to what I need to do for a positive confirmation of them being natural Co bearing spinel, and not a synthetic?
Curved Striae? Spectra? I need to go over on the GO.com forum, and see if they have info on it

 

[Nosean]

Hi,

fine material - they look like very fine hauyne - THE gem with the finest blue!!!

Check first RI - natural cobalt spinel has a low one around 1,710-1,715.

Cheap synthetic Verneuil have a higher RI course the Al2O3 ratio to MgO is not 1:1 - a bit more "corund" so higher RI - more than 1,720 is suspicious.

A problem are the more expensive synthetic flux spinel - similar range like the natural ones - so search for the iron line in the spectrum.

http://gemresearch.ch/grs-alert-a-new-type-of-synthetic-spinel-appears-at-the-bangkok-show/

Qualities like the small one you posted are tricky - you have the check EVERY STONE....

Hauyne is much simpler - and has the most beautiful blue!!!

I noticed that if not toptoptop color the Luc Yen cobalts shift in incandescent light ( Hauyne not...) ( I know...) to a purplish blue.

Could you tell me the size of the round lot?