The London Hammer is considered what type of artifact?
London Hammer:  Part of the hammer is embedded in a limy rock concretion, leading to it being regarded by some as an anomalous artifact, asking how an obviously man-made tool could come to be encased in a 400 million year old rock. Out-of-place artifact: An out-of-place artifact (OOPArt) is an artifact of historical, archaeological, or paleontological interest found in an unusual context, that challenges conventional historical chronology by being "too advanced" for the level of civilization that existed at the time, or showing "human presence" before humans were known to exist.
An out-of-place artifact (OOPArt)