Carbon subjected to intense pressure and heat far down in Earth’s interior becomes compacted and incredibly hard.
In this state it is called a diamond—the hardest known natural substance on Earth. Not only do diamonds possess great beauty, but also great utility.
The sparkle and brilliance of diamonds come from the way the light refracts through this hardened element. And its hardness makes it extremely useful in cutting virtually anything with great precision.
Zircons, often substituted for diamonds, can be made a rich blue by heating the stones in the absence of air.