To understand the formation of the rock we call slate, we must first travel back in time a few billion years. As volcanic lava flowed over the ground, superheated clay deposits mixed with ash. This mixture eventually dried in layers, much like shale but noticeably harder. Slate can be split into slabs, and those slabs can be split once again into thin sheets.
Slate has been used for a number of applications over the years. One common use is as a natural form of roof shingle. Individual shingles are manually carved from a master slab and sold to roofers and contractors. The tiles are arranged in offsetting rows over a few layers of tar paper and sealants. Slate shingles are desirable for their natural durability and heat absorption, but individual shingles can become fragile. Compared to other modern shingle materials, they can also be relatively expensive.
Tiles for walkways and gardens can also be made of slate. These tiles are generally thicker than roofing shingles, so they are not as likely to break or flake off. The tiles are known to be irregular in shape, which allows for some creative applications.
Another common use for slate is found in the recreational field. Because slabs can be ground down to an absolute flatness, slate is often used as the base for billiard and pool tables. A slab is custom-cut to the dimensions of a table and made as level as possible. A layer of green felt is then glued to the top of the slab to provide a playing surface. The slate tablet is by far the heaviest element of a pool table.
Perhaps the most common application of slate has been around since our childhoods. The relatively smooth but granular surface is ideal for use as a chalkboard. An old-fashioned chalkboard was generally black in color and noticeably heavier than today's greenboards. Individual students often carried small pieces of slate to demonstrate their skills. Tablets were also used by filmmakers to record vital information about filmed scenes. An assistant holding a board and a clapper would write the number of takes, the scene number, the title of the film and the director's name. The sound of the clapper would later be used to synchronize the sound with the filmed scene.