What is the primary composition of black chalk?

The primary composition of black chalk is carbonaceous shale, which is a sedimentary rock that contains a significant amount of organic carbon. Black chalk is characterized by its fine and soft texture, often used for drawing due to its ability to produce rich, dark lines.

In contrast to the other choices, calcium carbonate is primarily found in white chalk but not in black chalk. Compressed charcoal, while similar in application, does not represent the composition of black chalk; instead, it is made from burnt organic materials, leading to a different texture and consistency. Graphite, while it can be a component of some drawing materials, is distinct from black chalk, which has more organic material characteristic of shale. The unique composition of carbonaceous shale in black chalk allows it to hold a different aesthetic quality and applicability in art than the other materials mentioned.