Slate Stone

Slate is sedimentary, but unlike limestone which is shell and bones, slate it is most famous for its characteristic. Slate can also contain some of the same materials found in granite, which makes some slates iridescent and/or hard.
Like limestone and marble, the color comes from trace metals. The wild colors on most Indian slates are the result of splitting the slate along natural layers, which exposes the metals to the atmosphere and they oxidize (rust).