I am looking to buy two blue Spinels from a seller but I am unsure how to tell specifically if they are real or synthetic so I would like someone to give me some advice here. While its expensive, if I have to I will send them to GIA but I was hoping I could do some tests prior. Now the size of the spinels is what first leads me to believe they are synthetic because one is 40cts and the other one 45cts which is extremely large especially since they are internally flawless. One is darker than the other lighter, the lighter one has a bit of a steely blue look typical of blue spinels. It’s just the size that has me unsure. The seller told me that they would take them back if I found out they were synthetic (but I’d like to save myself the GIA fee if I could). They have been tested with the Presidium Duo detector as spinel but that doesnt tell you if they are synthetic or natural. If anyone could give me a brief overview of some tests (maybe some website links) I would really appreciate it! Thank you for your help!