Wikipedia
Blue ground
In reconnaissance for diamond mining, blue ground refers to a layer of non-oxidized kimberlite. Kimberlites are a class of igneous rocks commonly associated with diamond mining. Before the advent of modern geophysical probes, the ideal way to find a kimberlite pipe was to search for " yellow ground", a layer of oxidized kimberlite which is a deep tawny yellow. "Blue ground" was regarded with scorn during the South African diamond rushes, and one of the central characters in the initial diamond rush, Barney Barnato, made his fortune by purchasing digs in which the "blue ground" was all that remained.