I didn't find this in the hole where I dreamed there were pieces of eight. I was just kind of swinging the detector around while walking back toward the house from said hole, and ...
That's a nail, photographed next to a 100mm cigarette to show its size -- maybe 5" in length.
Minimal Internet research says that these were around as early as 1820 and started getting replaced by more modern nails in the late 19th century. So it's presumably at least 120 years old and maybe close to 200.
It was only a few inches under the surface, so I guess it might have been used elsewhere and then dropped in my yard much later, perhaps when a load of dirt was brought in or something. But for the moment, I'm going to assume that there was an old house, barn, whatever, 150-200 years ago, in what is now my "side yard."