Slate is a fine-grained rock that can easily be split into thin, durable sheets. It consists mainly of grains of mica and quartz, plus smaller amounts of chlorite, hematite, and other minerals. Most slate is grey to black in colour, but the rock may be red or purple, depending on its mineral content.

Slate is a metamorphic rock. Most slate is formed below the earth's surface by changes in the makeup and appearance of shale, a sedimentary rock. Shale consists of clay and fine particles of quartz. Heat from deep in the earth changes some of the clay in shale into mica and chlorite. Slate results when pressure created chiefly by mountain-forming movements in the earth's crust squeezes the mica and other minerals into parallel layers.

The building industry uses slate for roofing and flagstone because the rock is weatherproof and long lasting. Slate is also used to trim the fronts and lobbies of buildings.

Slate 1 :
Slate is a split stone and is one of the world's oldest natural products. Slate is available in various colours. Some shades will change colour on exterior use; therefore a non-fading shade should be specified for exterior applications. Water absorption of slate in accordance with ASTM C121.

Slate 2 :
There are items on the market that can be written on again and again. These are 'slates', I guess that word is right, that consist of a layer of transparent film, under this film there is a layer of magnetic dust or something but I don't know the exact make up of the materials used. Then one has a pen to go with that with which one can draw a line over the magnetic part of the board which will be black. When one wishes to erase the drawing/writing/sketch there is a little handle on the side connected to an elongated magnet under the magnetic film. When you move this handle from right to left then all the magnetic particles get evened out and the screen is blank again. A year ago I saw this on TV too, but this time as a tool to be used in offices as a substitute for the 'flip over', this in order to spare paper.

What you are describing sounds like the 'Etch a Sketch'. If you have ever tried it, actually writing on it is very difficult. There is a simple writing device which is a transparent gray film over a slick, black cardboard background. You write or draw on it with a blunt stylus. This causes the film to stick to the background, making a line. When you want to reuse it, you simply pull up one end of the sheet and it is 'erased'. For temporary drawing or writing, it would be hard to beat the simplicity of chalk and slate! Also available in multiple colors! Paper is still important, as it would be the lowest-tech medium which would have any chance of surviving for a long period of time. Writing should not be allowed to die out!

Slate 3 :
Slate is basically metamorphosed shale. Shale is made up of slit and clay that is similar in composition to sandstone except that the particle size is much smaller. The shale is subject to heat and pressure and the fine particles are "melted" together to form slate.

Slates are a natural cleft material and are typically used for flooring or roofing applications. Some slates are available in large slabs and can be used for counter tops.

Most slates are suitable for exterior and interior applications. Be careful when specifying green slates as their color may change or fade with time.