The form of calcium carbonate that is softer than marble but harder chalk is limestone. Limestone is a sedimentary rock composed mostly of calcium carbonate (CaCO3). It is softer than marble, which is a metamorphic rock formed from limestone, but harder than chalk, which is a soft, fine-grained sedimentary rock composed of calcium carbonate. Limestone is used in a variety of applications, including building materials, cement, and agriculture.