Slate
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Slate was formed in layers at the bottom of the ancient seas, then compressed and heated. It comes in a range of colours, including greys, greens, pinks and purples.
Slate is notable in having more natural variability than almost any other flooring material, both in surface finish and in thickness. This means that it is impossible to achieve a completely even finish with a slate floor. This may be an effect you want to achieve - but this can be a disappointment to some people.
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Slate can look wonderful in the right setting, but it is important that you are aware of the inherent variation if you choose it for your home.