Project U.F.O. 1970's TV Series / S02E08 "The Superstition Mountain Incident"

Amidst the anticipation of an imminent Solar eclipse, a High School student, and his girlfriend, observe a cylindrical UFO that appears to land inside a disused local South Dakota mine. Subsequently, they discover two geometric metal ingots. Air Force analysis of one of these objects reveals it to be composed of almost impossibly pure magnesium. After Ryan and Fitz ultimately wind-up their investigation, it is revealed that the magnesium ingots may be just the most recent instance of such Extra-terrestrial offerings. Joyce Brothers appears as a materials analyst hired by a TV tabloid journalist.