Dowsing maps is possibly better kept under your hat. It works but you'll be described as loony as it conforms with no practical theory.

Also black and shiny does not rule out sandstone. If the rock is sat on often it will become shiny and stones were painted by the ancients - red, white and black oftem. Ground coal may have been used as a pigment for black - it penetrates the grain.

(for TomBo) Blackstone has a sibling or two.