Sand is composed of all kinds of rocks and minerals, so its chemical properties greatly vary. Most sand is made of quartz, which is largely silicon oxide. Physically, sand is made up of tiny, loose grains of rocks or minerals that are larger than silt but smaller than gravel.
Quartz is a very hard mineral, ranking a 7 on the Mohs hardness scale. Pure quartz is transparent to translucent and the crystals are often hexagonal.
A lot of sand, especially that found on beaches, is made of basalt, an igneous rock extruded from volcanoes. Much of the crust of Earth's oceans is made out of basalt. Basalt is mafic, which means that it's made of iron and magnesium minerals, such as plagioclase and pyroxene. Other types of sand are made up of tiny bits of coral and crushed snail and clam shells.
Sand can also come in many colors. Some beaches in Hawaii are famous for black sand, whereas beaches in the Caribbean are famous for pink sand. Because sand is composed of so many materials, it is possible to study grains of sand under a microscope and discover where they are from and what they are made of.