Found this in the Preston PoundLand. I have put a ten pound note in the image so you can see roughly how big it is (21cm x 29cm). It's about 1cm - 2cm thick.
It's very heavy so I think it must be granite, it also says 'granite chopping board' on the label.
I bought the black one but there there was at least one other colour - a creamy colour with brown bits.