Nachi stone have been used for Japanese garden from Edo era(1603-1868).
Main producer of the stone is Kumano-shi, Mie prefecture.
Black Nachi stone has pure black color and shiny surface.
Black Nachi stone (kuro nachi)
White Nachi stone (shiro nachi)
The stone is used for rainwater drainage ditch(*1), pavement(*2) (used as is or sometimes used with cement ), and as a gravel stone on the bottom of water (*3).
*2 "Araidashi" exposed-aggregate finish