When I was a kitten, I loved collecting rocks and gems. So, I was very excited to talk about your question with my friend Johannes Haemmerli. He studies minerals in the School of the Environment at Washington State University.
Minerals are solids that form from non-living elements in nature. They have a very specific structure for how those elements are arranged. Haemmerli told me that nearly all gems are minerals or sometimes mixtures of minerals.
You’re right that gems form underground. A diamond forms when the element carbon is buried nearly 100 miles deep inside the Earth. It’s super-hot and there’s tons of pressure down there. Eventually the pressure pushes the carbon atoms together to form the mineral we call diamond. Above ground, where there is much less pressure, the same carbon can come together and form a mineral we call graphite. That’s the “lead” of your pencil.
Sometimes gems form in scalding hot water called hydrothermal fluid. This water is underground and has dissolved elements in it. As it cools, those elements form minerals like quartz.