Features that will help me detect this object?

I would like to detect an object in digital images (photos attached). It is a small nut.

Sometimes I can successfully detect it by looking for all particles in a certain size range, and then looking for one with solidity and perimeter values close to what I think it should have. Other times my code fails and mistakenly labels something else as the object of interest. Are there any simple geometrical features that could help me differentiate this object from blobs such as those shown in the two example images?

Thanks.

Successful detection:

Unsuccessful detection (mistaking a different blob for the object of interest);

Answer 1

[yanqi liu]

sir，may be the target must have one hole in center？

[Clay Swackhamer]

Yes, one large hole