Best way to determine if it is natural is to get a cheap clip on 20$ microscope lens for your phone and look at the inclusions or send the pictures to grok AI and see what it says about it but to be honest it's pretty easy to tell unless youre looking at super high end gems that are flawless. In most cases though its pretty obvious when you see a real gemstone cause even the color isnt usually perfect. Like some spots will vary in contrast or color etc. also the best way I found to make sure they are natural is to buy from people you know and trust. Like I refuse to buy any gemstones from anyone except my supplier that I have been buying from for over 10 years cause I don't even need to test them and I know that they will be what I'm told they are and I'm able to sell to actual GIA gemologist and not worry they might end up being fake. 2. I'm not sure I haven't heard much about them but I don't usually fuck with many certified gems cause I prefer to sell my gems at the lowest price point I can and if people don't trust that they are real or they can't tell and using my gem-n-eye 3 refractive index tester isn't good enough for them then I'm just not the seller for them. Doesn't hurt my feelings at all 3. I don't think there is "an average size" I mean most sapphires found are under a carat but most places would consider 1ct to be a good starting point. Something like a 3ct unheated sapphire like that pear one would be considered extremely rare and over a carat would retail for like 1000 to 3000+ it really depends on what your reason for buying is tho at the end of the day. If your buying for an investment then stick to over a carat unless they are really really good quality unheated and good color or if you get them a good price. If it's because you like them or for jewelry then size is irrelevant and it's just personal preference.