Quartz is one of the most common minerals on Earth, made of silicon and oxygen atoms locked together in a repeating pattern. If you've ever picked up a clear or milky white rock that felt surprisingly hard and heavy, you might have been holding quartz.

Pure quartz is clear like glass, but it often contains tiny impurities that give it beautiful colors. Purple quartz is called amethyst. Pink quartz is called rose quartz. Smoky brown quartz is called, well, smoky quartz. These colorful varieties have been prized as gemstones for thousands of years.

Quartz is incredibly useful beyond just looking pretty. It's one of the hardest common minerals, which makes it perfect for sandpaper and grinding tools. Even more remarkably, when you squeeze a quartz crystal, it generates a tiny electric pulse. This property makes quartz essential for watches and clocks: a quartz watch uses these electric pulses to keep incredibly accurate time. The next time you check a digital watch, you're relying on a sliver of quartz crystal vibrating 32,768 times per second.

Quartz forms a major component of sand on most beaches, and it appears in granite, the rock that forms much of Earth's continents.