Silvery-pink rock nipple
If you find a metallic or pearly red-to-pinkish disc adhered to the underside of a stone, you may wonder what it is. Or you may dismiss it--there's lots of stuff under stones, who cares, right? Oh someone cares, trust me.
Round bulges in the surface of the disc reveal its purpose. This is an egg-protecting device, silken and flush against the rock, difficult to remove or penetrate. It is made by a small spider that lives in such places--a spider with a body that somewhat resembles an ant, enough so to be lumped in with "ant-mimic ground spiders." The egg-case belongs to a spider known as "Phrurotimpus".
There are seventeen species in the genus Phrurotimpus in North America. According to some sources the name means "guardian of stones." My Composition of Scientific Words book verified "Phruro" to mean guard, but I was unable to find any etymology for "-timpus."
Many thanks as always to Charley Eiseman, whose book Tracks and Sign of Insects and Other Invertebrates has been an amazing revelation to me.