Chalk is a form of limestone that is distinguished by its relative purity and close, fine grain. Humans have been using it for a variety of tasks for centuries, taking advantage of its properties for things like writing on blackboards, marking out fabric patterns for cutting, and delineating game fields in sports like tennis and American football. This mineral is extremely abundant, and it is widely quarried all over the world.
The chemical formula for chalk is CaCO3. Its primary ingredient is ancient fossilized sea organisms, which were deposited and subjected to high pressure over centuries, so deposits often occur in areas that were once underwater. Rocks that form through the deposition of sediments and other materials are known as sedimentary rocks; chalk is probably one of the most well known examples of this type of rock, thanks to its common use. Impurities like quartz, feldspar, and silica are often found in the deposits.
Although people often think of chalk as a very soft mineral, it is in fact remarkably resistant to weathering. This trait led to the formation of the famous White Cliffs of Dover, as the elements wore away at other mineral materials in the large limestone deposit that streaks across much of Europe, leaving imposing deposits of chalk in the form of cliffs and sea stacks behind. The mineral is also very porous, and it tends to store groundwater extremely well, releasing it during drier periods.
The natural mineral can range in color from yellow to gray to white, depending on the impurities it contains. Depending on what it is going to be used for, it may be treated to remove impurities and refined so that it turns white. It can also be dyed with various coloring agents to produce things like sidewalk chalk.
Most classroom chalk today is not actually made from limestone, but rather from various other mineral composites. The powdered material that was once used to mark out sports fields has also since been replaced by various other minerals or paint. It is still possible to find a few commercial products made from chalk, and many natural history museums keep it on display since it is a mineral of interest to many people.