CWGC Gravestones

While out in the Wirral a couple of days ago (NJR, CDP, Prezzie and Myself), we stopped at St Marys Church, Eastham. There was a plaque in the church we wanted to visit but while we were there we had a look atthe churchyard and there were some CWGC Gravestones, some were made with the usual limestone but there were a couple of what appeared to be slate Gravestones too. Does anyone know why they were made of another material? Has anyone else come across any other non-limestone memorials?

I don't know why in this area they didn't use the usual limestone but I have seen it elsewhere.
I have a photograph of one at Holyhead, admittedly from outside of the time where CWGC was responsible, that is made from slate. The majority of gravestones in Scotland are made from Granite, though the south of the country seems to be limestone.

I've seen some white marble ones in Hertfordshire.

At the Naval Cemetery at Shotley, Suffolk there are some different headstones, the WWI are certainly different looking almost like concrete compared to the Limestone WWII + Post War headstones