What's the difference between a rock and a mineral?

Rocks and minerals hold precious clues about how Earth formed and evolved over billions of years — but what is the difference between the two?

In essence, rocks are aggregates of two or more minerals. Minerals, meanwhile, are solids that, with a few exceptions such as diamonds, lack carbon and are arranged in an orderly, repeating "crystal structure." 

"Minerals are basically the building blocks of rocks," Erika Anderson, an honorary curator of mineralogy and Petrology at the University of Glasgow's Hunterian Museum in Scotland, told Live Science. "It's kind of like atoms in a molecule, so minerals are the atoms." 

Each type of mineral has a unique crystal structure, which results from its chemical composition and dictates a set of physical properties, such as hardness, color or magnetism, according to the United States Geological Survey (USGS). For instance, halite — the natural form of sodium chloride (NaCl), from which table salt is made — is a soft mineral that forms clear, cube-shaped crystal fragments. Different minerals, like aragonite (CaCO3) and calcite (CaCO3), can have the same chemical makeup, but their crystal structure and physical properties differ because of how they each formed.

"For each mineral, they will have a set way that those atoms bond together," Anderson said. "Some minerals have the exact same elements in them, but they're bonded differently, so it makes them different minerals."

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A good example of a mineral is quartz, which is found across the world and in different rocks, such as granite and quartzite, Anderson said. Quartz is made of the chemical elements silicon and oxygen and has the chemical formula (SiO2). The mineral is colorless in its pure form, but impurities can either make quartz crystals appear opaque or stain them pink, purple, yellow or brown.