Rocks are continually recycled, typically on timescales of millions of years, in what's known as the rock cycle. The three major types of rocks - igneous, metamorphic, and sedimentary - can each be turned into a new metamorphic rock when given enough heat and pressure. Each rock type can be turned into a new igneous rock if it is completely melted and then cooled again. Sedimentary rocks are made when other rocks are broken into fragments and those fragments are cemented together to form a new rock. One of the most common types of sedimentary rock is sandstone.
As the name implies, sandstone contains sand-sized grains of rock fragments and individual minerals broken down from other, older rocks. We all have a sense of how big sand grains typically are from our experience going to the beach or playing in a sand box. Geologists also know that sand-sized grains have a particular measurement, from about 1/16th of a millimeter to 2 millimeters.
We say source rocks, the original rocks that eventually make up the small grains of a sandstone, are weathered when they break down. The pieces coming off a source rock can then be eroded, or carried away from the source area. The longer the chunks of a source rock are carried by wind, water, or ice, the more likely they are to be broken down into very small fragments. Certain minerals found within rocks are also more likely to survive significantly long rock fragment travels. Because quartz is such a hardy mineral, unlikely to undergo chemical changes during erosion, it is the mineral found in the greatest amounts in many sands. On most beaches (and of course there are exceptions), you will find a lot of hard, almost clear-looking grains of sand, which are most likely quartz.
After a source rock is weathered and eroded, the resulting sand grains might fill a bowl-shaped basin on land or underwater. With the pressure from more sediments landing above and the movement of water through the grains, the sand becomes sandstone. Finer grains of rock and new mineral growth fill the spaces between the grains, cementing the new rock.
