One of the chemical properties of calcium is that it reacts with a lot of other substances. As a result, pure calcium is never found in nature; it reacts too easily with other elements and with water.
Calcium belongs to the alkaline-earth metal elements on the periodic table, which all react with water. When calcium and water mix, they bond to form calcium hydroxide, and they release a hydrogen gas. This occurs regularly in nature. The bond happens on contact but is relatively slow, so it isn't a violent reaction. When it comes into contact with the air, calcium reacts and tarnishes, and a layer of calcium oxide forms between the oxygen in the air and the pure calcium. Calcium oxide and calcium hydroxide are both basic, as opposed to acidic.
Calcium reacts with carbon in nature as well, forming calcium carbonate, which is the main component of limestone. It is also present in marble, chalk, oyster shells and coral.
Another chemical property of calcium is that, when it burns, it makes a bright reddish-yellow flame.
Finally, most of the halogen elements will react with calcium simply by coming into contact with it, forming compounds. The exceptions are bromine and iodine; calcium only reacts with these two substances when heated. It also reacts with most of the non-metal elements, particularly sulfur and phosphorus.