The black stone is pure magnetite. The purpose of this prototype is discrimination that can indicates very deep targets such as for digging . So far all very deep targets were missed , as well as non-magnetic targets are indicated by discrimination like magnetic (iron ) . The prototype was tested on sand of pure magnetite and can be used to search for golden nuggets , while at the same time rejects the shallow iron objects . In tests on different soils shows very low soil noises , and almost complete absence of false signals . There is a video with the same prototype in youtube , from October 2018 .