Here are a few more things to think about (and remember, what we are working from to make this determination).
From what I can see, there appears to be the remnants of a 'rim'. Look closely. Also, if you look closely, you will see that the 'coin' is not perfectly round; but actually, a bit 'oval'. And that, is what would indicate it having been flattened.
I almost said that it could have been a nickel that someone had just rubbed down on cement or whatever, or filed down to be thinner.* But that would not account for the shape of it or any of the other indications which I see present. Besides this, look at the edges. They do not appear to be perfectly round either, there are small 'dents'.
In answer to what was said regarding 3c pieces getting dark, yes, they certainly can get dark. But that is not the only consideration. Perhaps what I had said about silver getting dark (instead of corroding like a nickel and copper coin) confused someone. I mentioned this in relation to the corrosion factor, not apart from it. There are generally certain things that we will find in combination with each other that will tell us what we want to know.
Again, this may be a good exercise in coin identification, something which we have an opportunity to learn something from. But as far as the 'coin' itself goes, it is probably of little value do to the condition that it is in. It may rate in the old standard of grading as a 'poor' at best - IF it can be identified at all. As it is, it is probably only worth the value of the metal that it is made of.
* IF it were a nickel which was simply filed down or otherwise worn down to be thinner than normal, it would most likely still register on most metal detectors as a nickel or in the 'nickel' range. Again, this is due to the same or similar metal content in conjunction with the diameter or 'spread' (area) of the coin within the coil range of the detector. That would be something like the difference on the detector between the same coin being detected at a slightly different depth - it would still register as or in the 'nickel' range. Best regards,
JDSCOIN