Organic rock, such as Marble and Sardonyx, come from living organisms. They are formed directly or indirectly from materials that were once living and are made up of mostly calcite and limestone. Calcite is colorless or white when pure, but may be of almost any color with varying shades of red, pink, yellow, green, blue, lavender, black or brown, owing to the presence of diverse impurities or sugar veins. It may be transparent, translucent, or opaque. Its luster ranges from vitreous to dull. Many crystals, especially the colorless ones, are vitreous, whereas sugar veins are properly referred to as granular masses, especially those that are fine-grained, and tend to be dull. Calcite is number 3 on the Mohs hardness scale; it can be scratched readily by a knife. Most fresh water and seawater contain dissolved calcium carbonate. All limestone is formed when the calcium carbonate crystallizes out of solution or from the skeletons of small sea urchins and coral. Limestone is a rock made of calcite.

Most limestone is gray, but all colors of limestone, from white to black, have been found. Deposits of limestone are typically formed from the shells of creatures such as clams and certain microorganisms. The shells collect on the ocean floor, accumulate and compress into rock over time. Some sedimentary rocks are formed when a sea or lake dries up, leaving large amounts of dissolved minerals that then concentrate and form solid rock. Marble is formed by the alteration of limestone by heat and pressure. The calcite in the limestone changes and fossils and layering in the original limestone disappear as interlocking grains grow. If the limestone is pure, a white marble is formed. Limestone may include layers of clay or sand, which may form the attractive sugar veins (colors) found in marble.