Any ideas what this is?

I dug this up last night in an area that should have old coins. Its 24mm one way, and 26mm the other, and is 1mm or less thick. It looked like copper, but gave a reading almost as a nickel would, and when I took a brush to it, the surface started flaking off. It looks like I can see the numbers 18 at the top of the thing, I say top, because the 8 has a larger circle on the bottom, but this really throws me off, because I can't find any coins with the date at the top? Any help is appreciated.
Thanks, Ed

P.S. I found it using my Sun Ray 5" coil, about three inches down, buried in gravel.