Meus error; if the reaction kinetics be (and they are) such that diamonds take longer than even geological lengths of time to turn into graphite at STP, then their lack of thermodynamic stability while sitting on the surface is not particularly relevant to a consideration of their geologic formation and destruction.
I believe diamonds were still being formed rectally in the Chicago area as late as the mid 1980s.
Source: Ferris Bueller’s Day Off.
There’s believed to be a diamond deposit in Indiana that has not been found:
From : http://legacy.earlham.edu/~scottna/EconomicGeology.htm
If I’m reading this thread right, pretty much everything you learned about diamond formation from Ferris Bueller is wrong.
This Forbes article summarizes a paper on some of the youngest known diamonds (the Nature paper itself is linked and not paywalled.) Here is a somewhat related interesting article on kimberlites. And just for the heck of it, the wiki page for carbanado, a type of diamond nobody knows where the hell it came from.
Let’s just point out that those “youngest” diamonds are the 1-billion year-old ones we’ve been discussing all along, since dolphinboy’s Wiki article cited in post #2 already took them into account. They’re still hundreds of millions of years older than the kimberlites they came up with.
And geologically, carbonado is a rock that contains diamonds, not a mineral in its own right.
You are correct, sir. Ha!
Interesting new study on kimberlites:
https://www.science.org/content/article/death-supercontinents-brings-diamonds-surface