A diamond is a mineral made of carbon atoms arranged in a crystalline structure. It is the hardest known natural substance, with a Mohs hardness of 10. It has a refractive index of 2.42 and a high dispersion of light, giving it its characteristic sparkle. Diamonds are often colorless but can also come in various colors, including pink, blue, and yellow. They are also known for their high thermal conductivity and ability to withstand extremely high pressures.