Definition of slates:
(n) :
a fine-grained homogeneous
sedimentary rock composed of
clay or volcanic ash which has been metamorphosed so that it cleaves easily into thin layers; the
bluish-
grey colour of most slate
"put it on my slate - I'll pay you next week."
(adj) :
(colour) Having the bluish-grey/gray colour/color of slate
"the play was slated by the critics."