Sand is a granular material composed of finely divided mineral particles. Sand comes from many locations, sources, and environments, and it forms when rocks break down from weathering and eroding over thousands and even millions of years. The composition of sand varies, but it is defined by its grain size, which is smaller than gravel and coarser than silt. Here are some of the materials that can make up sand:

• Quartz: Much of the worlds sand is made out of tiny crystals of the mineral quartz, which is made out of silica and oxygen, the two most common elements in Earths crust.

• Feldspar: Feldspar is another common mineral found in sand, which is brown to tan in its original form.

• Iron oxide: The tan color of most sand beaches is the result of iron oxide, which tints quartz a light brown, and feldspar, which is brown to tan in its original form.

• Volcanic material: Black sand comes from eroded volcanic material such as lava, basalt rocks, and other dark-colored rocks and minerals, and is typically found on beaches near volcanic activity.

• Calcium sulfate: Sand may be composed of calcium sulfate, such as gypsum and selenite, as is found in places such as White Sands National Park and Salt Plains National Wildlife Refuge in the U.S.

• Organic matter: Less common but no less inviting beaches, devoid of quartz as a source of sand, rely on an entirely different ecologic process. For example, the famous white-sand beaches of Hawaii come from the poop of parrotfish. The fish bite and scrape algae off of rocks and dead corals with their parrot-like beaks, grind up the inedible calcium-carbonate reef material (made mostly of coral skeletons) in their guts, and then excrete it as sand.

In summary, sand is made of rocks eroded by rivers, the skeletons and shells of invertebrates, as well as calcium carbonate from parrotfish poop. The composition of sand varies, but it is defined by its grain size, which is smaller than gravel and coarser than silt.