Marian plums are an interesting fruit. They are certainly plum-sized, but their dusky yellow-orange color is unlike any other plum you may be familiar with. In fact, these tropical fruits are a relative of the mango.
Marian Plums on display in a Bangkok market
In Thai, they're called mayong-chit, and although they're native to Southeast Asia, I don't remember seeing them in the markets until a few years ago. the fruits are now rather popular, although they are rather expensive by Thai standards, and only available during a short season of around January to March.
The skin, which is edible, is surprisingly 'crunchy' while the fruit itself has a sweet and sour taste, which like mangoes can sometimes taste a bit like peaches.