Limestone is by definition a rock that contains at least 50% calcium carbonate in the form of calcite by weight. All limestones contain at least a few percent other materials. These can be small particles of quartz, feldspar, or clay minerals delivered to the site by streams, currents and wave action. Particles of chert, pyrite, siderite, and other minerals can form in the limestone by chemical processes.

The calcium carbonate content of limestone gives it a property that is often used in rock identification – it effervesces in contact with a cold solution of 5% hydrochloric acid. See our article about the “acid test” for identifying carbonate rocks and minerals.