Quartz is a mineral that's usually clear or white, but is sometimes tinted a translucent color. It's common for quartz to be part of other rocks, and often you can see its stripes cutting through smooth beach stones.
Quartz is very hard and shiny, and it's also quite common. There are many different types of quartz, and the rarest ones are considered semi-precious gemstones, like citrine and amethyst. This is the quartz that's used to make jewelry. Its chemical name is silicon dioxide, and it has a crystal structure. Quartz comes from the German Quarz, "rock crystal."