Looks to be immature or poorly pollinated pomegranate. The second picture shows a pomegranate in the upper left.

Ahh. the last picture (initial post) looks like a pom also. This last pic you posted should get some answers from the more experienced posters. Where abouts are you?

It looks like Jamun (Syzygium cumini ), also called "Indian Blackberry" or "Java plum". These trees are sometimes planted by immigrants from India. The fruit has a very "smoky" taste and is very high in tannins. Most Americans find the fruit distasteful, but many Indians have fond memories from their childhood eating the fruits off a Jamun tree in the summer. The fruit has a large seed inside, is very messy and the juice stains. Adding just a sprinkle of salt can supposedly help bring out the sweeter flavor.

If the shape seems distantly reminiscent of pomegranate, it might be because the two happen to be in the same order Myrtales.