Chalk is a form of limestone which is distinguished by its relative purity and close, fine grain. Humans have been utilizing chalk 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, so chalk deposits often occur in areas which were once underwater, as the mineral forms through centuries of deposition of dead sea creatures at very high pressure. Rocks which 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 a sedimentary rock, thanks to its ubiquitous use. Impurities like quartz, feldspar, and silica are often found in chalk deposits.

Although we 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 chalk deposit which streaks across much of Europe, leaving imposing deposits of chalk in the form of cliffs and sea stacks behind. Chalk is also very porous, and it tends to store groundwater extremely well, releasing it during drier periods.

Natural chalk can range in color from yellow to gray to white, depending on the impurities it contains. Depending on what chalk is going to be used for, it may be treated to remove impurities and refined so that it turns white. Chalk is also dyed with various coloring agents to produce things like sidewalk chalk and colored chalk for use in classrooms.

You may be disappointed to learn that most classroom chalk today is not actually made from chalk, but rather from various other mineral composites. The powdered chalk which was once used to mark out sports fields has also since been replaced by various other minerals or sometimes paint. However, 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.