If copper, thats cool, but doesnt look like the size of a penny, usually they are punched and are either missing or have extra material because of the blank.
1st - I noticed this was your very first post - so, Welcome Aboard Purplesky! You didn't list your state (or country) in your profile. So, you might consider jumping over to Sub-Forum: Select Your Area.... for information (i.e., clubs, hunts, finds, legends, maps, etc.) directly related to your state (or country).
I agree with the others. The dime is toned dark, stained or discolored by exposure to the environment. Clad dimes can turn dark quickly on saltwater beaches before they corrode. I fed five of these dark dimes dug on a beach into a coin machine yesterday, so they can be found discolored in circulation, returned to circulation after having been dug.
If your dime were missing clad layer, both sides, it should weigh-out lighter than a conforming dime.
As others suggest, this is incredibly unlikely but the test is super simple. Weigh it, weigh a normal dime, and compare.