Slate is metamorphic rock. However, instead of forming from a pre-existing limestone (like marble), slate is formed from the low-grade metamorphism of the sedimentary rock shale. Slate, like shale ("mudstone"), is a very fined-grained rock of mostly microscopic clay minerals with some microscopic quartz and calcite. Slate can also contain some of the same minerals found in granite, which make some slates iridescent and/or hard. The alteration of shale by heat and pressure produces the pronounced partings (cleavage) that give slate its characteristics. Like limestone and marble, the colour comes from trace metals. The wild colours on some slates are the result of splitting the slate along natural layers, which exposes the metals to the atmosphere, causing oxidisation.