Stack Exchange network consists of 175 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.
My mom thinks it's a Mirabelle plum tree (Prunus domestica), but those appear to produce orange plums. It's in southeastern Idaho, gets full sun from about 9 AM until sunset.
What do you think?
(Click to enlarge pics)
Immature Mirabella is the probable answer. I'm posting it here to clear this question from the unanswered list.
Other possibilities based on the picture would be Damson plums or immature Satsuma plums (image).
Required, but never shown