I haven't ever lost a fossil pearl before, but I have an idea how you might find it. As soon as you can, get a strong portable UV light or an electric model with an extension cord. (A regular poster type black light won't work) I don't know which would work better long wave or short wave, but some lights have both. Go out after dark, the darker the better, and sweep the light back and forth along the area you traveled when you went to talk to the neighbors. You will probably need to keep the light within two or three inches of the ground for the best effect. The pearl should glow yellowish or whitish under the UV light. Do a quick check on the pearl you still have to figure out the best distance for the light and the color to expect from the lost pearl.