Blackboard chalk is a calcium-rich material. Calcium on its own is actually a soft, silvery-coloured metal. However, calcium only occurs in nature as part of compounds. The most common calcium compound is a material called calcium carbonate.
Most people know calcium carbonate as rock, such as chalk, limestone and marble.
They may have admired coral reefs without realising that both the coral rock and the tiny green plants that live with the coral, known as algae, have calcium in them. Calcium even contributes to the pearls that are used to make rings and necklaces. When gardeners use crushed limestone, crushed bone meal, or pulverised shell as a soil-improver in their gardens, few may realise that the bones of animals as well as the shells of snails, mussels and myriad other creatures all contain calcium. The bones of people also have calcium compounds in them. In fact, about one-fiftieth of the human body is made from calcium.