Slate is a very fine-grained type of stone; the stone is created by sedimentation and consists mainly of gravel, sand, clay and sedimentation of animals and plants. Slate are often divided into 2 groups; quartzite/mica slate and slate made of clay. A Quartzite/mica slate has a great strength and is able to withstand acidic stress, and can on this point be compared to granite. On the contrary when talking of slate made of clay, these are much more soft and will most of the time get bleached by acidic stress. A slate made of clay is created over 80 million years ago, and quartzite/mica slate are created over 500 million years ago.