Slate rock is an example of a sedimentary rock. It gets formed from either clay or volcanic ash that has been superheated and pressurized underground. Slate is an extremely dense stone that comes in five subtle colors: green, red, gray, purple and black. Slightly less common are variegated purple and mottled purple slates, which have visible veins and shades of contrasting colors. Slate slabs are great for a variety of purposes.