If he mistook the "diamond" for one he had lost, and picked it up and sold it, he'd be factually mistaken, but not legally guilty.
However, he knows it's a "diamond" he has no prior connection to and he knows it was on his mother's property. Any belief he could have that it "belongs" to him is because of a legal error, not a factual mistake, and would therefore be legally indefensible.
In all likelihood, any legal action about this dumb thing would just result in him having to give it back, though, unless a vindictive prosecutor really hated his guts, which might be the case. I mean, if you knew the guy, wouldn't you?