Thread: Help I.D. this hone.

Came across this hone on a second hand tool store site. Advertised as green hone/oilstone for the princely sum of £16.00 delivered. Photos aren,t great ,but it is a light greenish blue colour ( aquamarine sprang to mind ).Surface glassy to touch and sprinkled with small black speckles . I think it appears to be some kind of slate . Any help would be appreciated in determining what hone this is .