Diamond is a crystalline form of pure carbon, organized in an extremely compact structure, which gives it exceptional hardness (10 on the Mohs scale). It forms deep within the Earth's mantle and then rises to the surface through rapid eruptions of rocks such as kimberlite. Its unique brilliance comes from its high refractive index and strong dispersion, which breaks down light into multicolored "fire." A diamond is not always colorless: there are also "fancy" diamonds that are yellow, blue, pink, green, or brown. These colors come from traces of elements or internal flaws: nitrogen can yellow the stone, boron can make it blue, and certain natural irradiations can produce green. Internal inclusions (crystals, microfractures) are like tiny archives that help us understand the origin and formation of the diamond. Although very hard, diamond can crack along cleavage planes if struck in the wrong place. There are natural diamonds and lab-grown diamonds, chemically identical but with different growth patterns. Finally, some diamonds are treated to alter their color, hence the importance of a clear description.