East of the California Sierras is the home of the oldest living plants on earth: Bristlecone Pines. But these intriguing fossils, with their unmistakable concentric circle patterns, are not products of ancient pine trees. Instead, grains of sand or other sediment rolling around on an ancient sea floor gradually picked up layers of deposits that show up as spherical cross-sections, called oolites, and that's how these fascinating stone were formed. The moniker "Bristlecone Pine" might have something to do with where these fossils were found, or else, their imagery might call to mind pine seeds. In any case, the more valuable stones are those with consistent patterns of circles, while less valuable pieces contain some irregularities in the pattern. All are completely natural, and all have unpolished backs.