Coral sand particles are primarily a form of aragonite, a calcium carbonate mineral. Aragonite is the crystal form commonly produced by reef-building organisms and preserved when their skeletons are broken down.
Despite its name, coral sand is not made exclusively from pieces of coral. It can include fragments from corals, calcareous algae, foraminifera, molluscs, and crustaceans. Parrotfish and other bioeroders grind reef material into sand-sized grains, while waves and currents further sort and transport them.
Aragonite is chemically the same compound as calcite—calcium carbonate—but its crystal structure differs. That distinction matters because aragonite is generally less stable at Earth’s surface and can dissolve more readily in acidic water. This is why coral sand fizzes or dissolves in acid tests, unlike quartz sand.
A common mix-up is treating all white beach sand as quartz. Tropical reef-island sand is often carbonate-rich instead, and its mineral composition reflects the biology of the surrounding ecosystem as much as ordinary physical erosion.