Scarabs, symbols sacred to the Egyptians, when worn were thought to bring good luck and ward off evil. The amulet's underside would be engraved with a motto, name, wish, or prayer.
Rather than carved stones, it became popular in the Victorian era to wear real scarab beetles. This seems creepy but their irridescent blue green coloring is actually quite beautiful.
14k gold ring with rose quartz (shown)
prong setting approx. 3/8" wide by 5/16" long