The Coso Artifact

While mineral hunting in the mountains of California near Olancha during the winter of 1961, Wallace Lane, Virginia Maxey and Mike Mikesell found a rock, among many others, that they thought was a geode - a good addition for their gem shop. Upon cutting it open, however, Mikesell found an object inside that seemed to be made of white porcelain. In the center was a shaft of shiny metal. Experts estimated that, if this was a geode, it should have taken about 500,000 years for this fossil-encrusted nodule to form, yet the object inside was obviously of sophisticated human manufacture. Further investigation revealed that the porcelain was surround by a hexagonal casing, and an x-ray revealed a tiny spring at one end, like a spark plug. There's a bit of controversy around this artifact, as you can imagine. Some contend that the artifact was not inside a geode at all, but encased in hardened clay. The artifact itself has been identified by experts as a 1920s-era Champion spark plug. Unfortunately, the Coso Artifact has gone missing and cannot be thoroughly examined. Is there a natural explanation for it? Or was it found, as the discoverer claimed, inside a geode? If so, how could a 1920s sparkplug get inside a 500,000-year-old rock?