It could be a plum........could well be something else. Without photos of the tree and the fruit (both whole and cut in half), we are just making wild guesses.

Snapped a couple pics. It almost looks like some kind of very late-ripening miniature nectarine. Fruit has been that color and size for a month but they are just barely larger than a golf ball.

Certainly looks like a nectarine. The leaves are right too. But since it's so small could it be a Nectaplum? Never seen one myself.

All I know is the fruits are hard as a rock,probably wont be ripe until september. And smooth as glass. I'm leaning towards some kind of weird wild plum. Maybe I'll know more when I see the seed/pit.

The leaves imply peach/nectarine, not plum. That's why I suggested an intergeneric hybrid. Being hard isn't an identification trait. They're all hard to start with. To see the pit just cut one in half. Plum stones are very different from peach/nectarine but I don't know what hybrid stones look like.

Do you recall the flowers? Plum flowers are white. Peach/nectarine are pink.

Another vote for nectarine.

I just cut one open and the small pit is very rough,like a peach pit. Some of the fruits are falling to the ground even though they are still hard. Apparently the tree thinks they are ripe. I ate a tiny piece (probably dangerous) and it tasted like an unripe apple or peach. Leaves are shaped like a peach tree but half the size of my Ranger peach tree leaves. I think it's just some mystery tree that grew from a wild seed.

It is normal for trees to drop a percentage of immature fruit. Known as 'June drop' here. The tree doesn't think they're ripe, it's just conserving its energy.

What's confusing me is the fruits have been full colored up for a month,are hard as a rock and smooth as glass outer skin. I think nectarines are grafted and this tree dont appear to be grafted. Weird.

apricot hybrid. I dunno - there has been a lot of intergeneric breeding - pluots and the like. I have a weird almond/peach cross which is quite horrible..

LOL I think this could be a lesson to someone thinking about growing a peach tree from seed,you dont know what kind of tree you're gonna get. These things aren't even fit for the hogs.

They could well make a good jam or pie even if unpalatable raw. Try cooking a few and see if they'll make a tasty compote. If they do they should be ok to use in the kitchen.