I am not sure about your hone. I do have few marked YLs but none of them is purple. They are different shades of grey/black. Also in the area of YL mine I have not seen this purple colour slate. However there is some indication that there were two quarries for YL and they are few miles apart and I have to say I wasn't anywhere near the other one. With DT it is a different story as it is cut from material originating in different mines. Those green dots are not uncommon in N. Welsh slate. I got one big enough to cut it into a hone. Those who were last year at London Razorcon had a chance to see it.