You’ve heard of carbon copies, but what about carbon opposites? Diamonds and graphite are both made of pure carbon, but like a hero and a villain played by the same actor in a B movie, they’re almost comically different.

Diamonds are transparent, prized for their clarity and brilliance. Graphite is opaque and nearly black, valued for its ability to leave marks—which is why it’s used as pencil lead.

Diamonds are famously hard. This makes them highly abrasive, and used on saw blades, drill bits, and sandpaper. Graphite is incredibly soft and is so nonabrasive that it’s commonly used as a lubricant.

Diamonds are an excellent heat conductor, while graphite can be used as an insulator.

How could two minerals from the same element be such different characters? The answers lie in where they were born and how they were formed.

Diamonds are created deep within Earth under intense heat and pressure. Their crystal structure is bonded tightly together in three dimensions. Graphite is formed from organic matter when it’s heated during continental plate collisions. Its mineral structure is in sheets, which are bonded loosely together and can slip past one another.

Different minerals made of the same element are called allotropes and are more common than you’d think. There are other natural and man-made allotropes of pure carbon. All have different characters depending on their crystal structure—and one of them will likely change our lives. We’ll talk more about it on a future EarthDate.