A rock known as tufa is a limestone formed by evaporation at a hot spring, lake shore, or other area. 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, clay minerals, pyrite, siderite and other minerals. rock known as tufa is a limestone formed by evaporation at a hot spring, lake shore, or other area. 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.