I have read that the distinguishing feature is that natural diamonds have flaws. Entrapped dirt or what have you that is not found in the sterile environment of the synthetic ones.
Maybe a specialist could tell a natural, formed over thousands of years, flaw from a lab-grown flaw, but really I think it's more like you can get a better grade from a lab (and especially with consistency, frequency, at a much lower price due to being rare to find naturally occurring). i.e. you can get flawed and coloured lab-grown diamonds too. (A mix of taking less care/time, older equipment, and binning the results like silicon wafers I assume.)
I worked for QVC a million years ago and the Diamonique they sold at the time was super clear and bright compared to authentic diamond.
I bought a yellow lab grown diamond for my wife's wedding ring and with a jewelers loupe you can see the flaws in it. It looks like dirt.
Like honey, the inclusions are the only thing that distinguish it from counterfeit.
Rubies and sapphires are just different flavors of impure corundum.
Well, I am definitely one of the 10,000 because I just started reading about the counterfeit honey market.