Here you go:

https://www.snopes.com/news/2022/11/17/elon-musk-emerald-mine/

here it is. The whole thing seems rather murky and somewhat shady too. The most specific statement on it is this:

Arnold's story included answers from Errol about the mine, who said he had "very limited involvement" in it. Arnold paraphrased his answers, publishing that he said "it was a handshake deal with a Panamanian man that resulted in something like 110 emeralds up front and then a semi-regular trickle of rough stones over a few years following," and that "there was never formal ownership."

The wealth earned from the investment in the mine was quantified to be about $400,000 USD, taking into account inflation when Arnold's piece was written in 2021, according to Errol. "Errol also quoted a lower number to me over the phone," Arnold wrote. "He also claims he was getting emeralds from other sources, with only 60-70% coming from this venture."

So maybe not a whole emerald mine, but like an unofficial small stake in an emerald mine? Still, 400k ain't bad at all for a handshake deal.