Slates, as mined so extensively around nearby Ffestiniog, were formed by relatively low-grade metamorphism of shale (fine-grained sedimentary rock). Coarser-grained sedimentary rocks exposed to much the same circumstances were often metamorphosed into schist. The minerals separated out into thin crystaline layers; mica plates can give a characteristic shiny appearence. Which the camera has failed to capture. Never mind.