Sand is mostly made of tiny fragments of rock and mineral, broken down over thousands to millions of years by weathering, water, wind, and even living creatures. The most common ingredient by far is quartz, a hard mineral that resists breaking down chemically, which is why it survives the long journey from mountain rock to beach grain. But sand can also come from crushed shells, ground-up coral, volcanic lava, and even evaporated minerals, depending on where in the world you find it.
What Sand Is Made Of
By strict definition, sand is any loose grain between 0.0625 millimeters and 2 millimeters across. That’s the range set by the Wentworth scale, which geologists use to classify sediment. Anything smaller is silt; anything larger is gravel. So “sand” is really a size category, not a single material. That said, most sand on Earth is overwhelmingly quartz, the second most abundant mineral in the planet’s crust. In typical continental sand deposits, quartz makes up around 95 to 96 percent of the grains, with small amounts of feldspar and rock fragments filling out the rest.
Quartz dominates because it’s exceptionally hard and chemically stubborn. Most other minerals in rock dissolve or crumble long before they reach a beach or riverbed. Feldspar, for instance, is actually more common in the Earth’s crust than quartz, but it breaks down relatively quickly when exposed to water and weak acids in soil. Quartz survives, grain after grain, accumulating wherever water or wind deposits it.
How Rocks Break Down Into Sand
The journey from solid bedrock to loose sand grains involves two broad categories of weathering: mechanical and chemical. Mechanical weathering physically cracks and shatters rock. Chemical weathering dissolves or alters the minerals inside it. In most environments, both work together.
One of the most powerful mechanical processes is frost wedging. Water seeps into tiny cracks in rock, freezes, and expands with enormous force. When it melts, the water flows deeper into the widened crack. Repeated freeze-thaw cycles slowly pry rock apart. A similar process happens with salt: in coastal or arid environments, evaporating water leaves behind salt crystals that grow inside rock crevices and push them open.
Pressure expansion is another major force. Rock that formed deep underground, under intense pressure, cracks and peels apart in layers when erosion exposes it at the surface. The sudden drop in pressure causes it to expand rapidly. Plant roots do their part too, growing into fractures and gradually widening them over years. Even burrowing animals contribute by loosening and displacing rock and soil.
Chemical weathering targets the minerals themselves. Water, especially when slightly acidic from dissolved carbon dioxide in the atmosphere, reacts with silicate minerals in rock. This process, called hydrolysis, transforms minerals like feldspar into soft clay while releasing dissolved silica and other elements. The rate of chemical weathering depends on temperature, how much water is available, and how much fresh mineral surface is exposed. In rapidly eroding mountain ranges, fresh rock surfaces are constantly revealed, keeping the weathering cycle going.
How Water and Wind Shape Sand Grains
Once rock fragments break free, rivers, glaciers, and wind carry them away. This transport doesn’t just move grains; it transforms them. As particles tumble downstream in a river, they collide with each other and with the riverbed, gradually rounding off their sharp edges and shrinking in size. Research on the Ganjiang River in China found that medium-sized sand grains become progressively rounder and finer the farther downstream they travel. Abrasion is the main driver of this rounding, while hydraulic sorting (faster water carrying away lighter, smaller grains) controls how grains get finer.
Wind does similar work. In desert environments, wind-blown grains slam into each other and into rock surfaces, smoothing their shapes and frosting their surfaces. Grains that have traveled long distances by wind tend to look distinctly different under a microscope than those carried by water.
Sand From Living Things
Not all sand starts as rock. On tropical islands, most of the sand comes from biological sources: broken shells, bits of coral, fragments of sea urchin spines, and the skeletons of tiny marine organisms called foraminifera. In places like Hawaii, where there’s very little rock washing in from rivers, nearly all the sand is biogenic.
Parrotfish are one of the most surprising sand factories on the planet. These reef fish have fused, beak-like teeth that scrape algae off coral. In the process, they bite off and swallow chunks of coral skeleton. A second set of tooth plates in their throats grinds the coral into a fine paste, which passes through their digestive system and comes out as white sand. A single large adult parrotfish can produce over a ton of sand per year. Scientists estimate that parrotfish are responsible for up to 70 percent of the white sand on beaches in the Caribbean and Hawaii.
Why Sand Comes in Different Colors
The color of sand is a direct clue to what it’s made of and where it came from. The classic tan or golden sand of most beaches is quartz, sometimes tinted by iron oxide coatings on the grains. Bright white sand can be either pure quartz with no iron staining or ground-up coral and shells, which are calcium carbonate.
Black sand forms on volcanic islands where basaltic lava, which is naturally dark, breaks down rapidly at the shoreline. Basalt is the most common source of black sand, though volcanic glass also contributes jet-black grains. Hawaii, the Canary Islands, and Iceland all have striking black sand beaches formed this way.
Pink sand gets its color from the crushed shells of tiny marine organisms called foraminifera, which have reddish-pink shells. Bermuda’s famous pink beaches owe their hue to one particular species. Green sand beaches, found in only a handful of places worldwide, are colored by olivine, a green mineral found in basaltic rock that’s dense enough to concentrate on certain shores.
Heavier, rarer minerals add variety too. Grains of magnetite (black and magnetic), garnet (red to pink), rutile (dark brown), and zircon (clear to brownish) all show up in sand. These heavy minerals originally formed in igneous and metamorphic rocks inland, then got concentrated by waves, rivers, or wind that carried away the lighter grains and left the dense ones behind.
Unusual Sand: Gypsum at White Sands
One of the most striking examples of uncommon sand is White Sands in New Mexico, where brilliant white dunes stretch across nearly 600 square kilometers. This sand is made of gypsum, a soft mineral that normally dissolves in water and gets carried to the ocean long before it can accumulate as sand. White Sands exists because of a geological quirk: water carrying dissolved gypsum flows into a basin with no outlet to the sea. The water evaporates, leaving gypsum crystals behind. Over many years, freezing, thawing, and wind have broken those crystals into billions of tiny sand-sized grains.
Gypsum sand is rare precisely because the conditions that create it are rare. Almost everywhere else, gypsum simply dissolves and washes away. White Sands is one of the few places on Earth where the geology and climate conspire to trap and preserve it.
How Long It Takes to Make Sand
There’s no single answer to how long sand takes to form, because it depends entirely on the source material and the environment. Volcanic sand can appear within years of an eruption, as waves and rain rapidly fragment fresh lava flows. Parrotfish produce biogenic sand continuously, replenishing tropical beaches on a timescale of years to decades. Quartz sand from continental rock, on the other hand, can take hundreds of thousands to millions of years to form, as weathering slowly dismantles granite and other hard rocks and rivers carry the surviving quartz grains to the coast. Some grains on a beach today may have been recycled through multiple rounds of burial, compression into sandstone, uplift, and re-weathering over hundreds of millions of years.