Skygems,
Yes, I am aware of your points, hence why I included my disbelief that it is a natural blue spinel specifically because of the size - I have never seen a spinel that size OR that intense blue as it was. After doing a bit more research I actually saw something with the presidium duo detector that is leading me to believe it is actually blue TOPAZ due to them being right above one another on the detection scale. This leads me to believe that the stone was actually a large blue topaz rather than a blue spinel. Given the size and clarity of the stone this definitely would make a LOT more sense to me. Finding ONE blue spinel that size is already remarkable but the seller actually had TWO that large which is even more rare, again making me doubt the genuine material.
Thank you all for your helpful suggestions, I will actually use all of these in my upcoming gem buys as I have been very hesitant to dump a bunch of cash unless I have a bit more tests to run to determine synthetic vs natural. I really appreciate everyone’s help.