What are the natural source of sand?

It abundantly occurs in nature and is formed by the decomposition of rocks. Sand particles consist of small grains of silica (SiO2). It forms a major ingredient in concrete, lime mortar, cement mortar, etc.

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And another question, do all grains of sand look the same?

Every single grain is different. There are no two grains of sand alike in the world. After being repeatedly tumbled by action of the surf this spiral sand grain has become opalescent in character. It is surrounded by bits of coral, shell, and volcanic material. Moreover, what are the principal sources and sinks of beach sand? These cells are geographically limited and consist of a series of sand sources (such as rivers, streams and eroding coastal bluffs) that provide sand to the shoreline; sand sinks (such as coastal dunes and submarine canyons) where sand is lost from the shoreline; and longshore transport or littoral drift that moves

How many grains of sand are in Bondi beach?

This means 1 square metre of beach, down to a depth of 3m should hold about 163 billion grains of sand. How big is a million grains of sand? A medium-sized grown-up woman has about 18 square feet, and a man has more than 20 sq ft. Each grain is 1/1,000th of a foot wide, so it takes 1,000 x 1,000 or about 1 million grains of sand to cover just 1 square foot.

Keeping this in consideration, are there really more stars than grains of sand?

The numbers pretty much matched. There are about the same number of stars in the observable universe as there are sand grains in all of Earth's beaches.