Slates are produced in a range of different colours, ranging from pale blue-grey to almost black. Green, red and purple are also readily available. Many factors affect the colour; for example, mineral composition, grain size or the environment of deposition. Slates containing a high proportion of chlorite are green, while those containing a high proportion of quartz and white mica are pale grey. Fine-grained slates are usually darker in colour than their coarse-grained equivalents. However, the greatest impact on colour is the environment in which the original sediments were deposited and underwent diagenesis (the process of converting sediment into a rock).
The calm water necessary for the deposition of mud (the precursor of slate) is often associated with stagnant, anoxic (lack of oxygen) conditions. Under such conditions, organic matter is not completely oxidised to carbon dioxide and water but leaves a carbon residue that becomes graphite when the rock is metamorphosed. The greater the amount of graphite present the darker the colour of the slate.
The iron ore mineral, pyrite, is formed under similarly anoxic condition, hence there is a strong association between the presence of graphite and pyrite in slate Due to the anoxic conditions, the sulphate ion, present in sea water, loses oxygen to become a sulphide and combines with iron to form pyrite.
In contrast to the graphitic and pyritic slates described above, oxygen is readily available in slates formed from muds deposited on the sea floor by turbidity currents. These underwater currents, caused by increased density due to a load of sediments in suspension, travel downwards at great speed depositing their load as they spread out over the sea floor. The coarser-grained material is deposited first, then eventually the fine-grained mud, the precursor of slate, is deposited on the sea floor. As these deposits (turbidites) are laid down in relatively oxidising conditions, any iron ore mineral present is the oxide haematite, Fe2O3. A very small concentration of haematite will give the slate a reddish colour. The most distal (furthest from the source) of these deposits are very fine-grained and have a deep red colour. The best quality slate is produced from these very fine-grained red deposits.