Where does sand come from?

Mostly from mountains, by way of rivers, over an extremely long time. Rock breaks into smaller and smaller pieces, water carries them toward the sea, and what finally arrives is sand.

Sand is sorted by size rather than by what it is made of: too big to be dust, too small to be a pebble. Pale beaches are often mostly quartz, because quartz is the traveller that survives the journey when softer minerals crumble away.

Some sand has a stranger beginning. On one reef in the Maldives, more than eight of every ten new grains had passed through a parrotfish, which scrapes coral with its beak and lets the rest go.