My 4 yr old daughter found this tooth/stone on the outskirts of an industrial estate near fields in Suffolk. Is it a tooth or just a tooth shaped stone? It looks a bit like flint colour underneath but there is a hollow inside?
It is definitely flint, which means it is unlikely to be a tooth (partly because of the material).
Flints sometimes reflect fossil sponges in one way or another, and I think yours falls into this category. The texture on the inside surface and the external shape are both suggestive (no more than that) of sponges.
As with many flints, the best we can do is make an educated guess; there is often simply not enough evidence to make a firm ID or comment.