Diamonds are crystalline forms of carbon (C) arranged in a cubic crystal structure, making them the hardest known natural material with a Mohs hardness of 10. Their exceptional hardness, high refractive index (2.42), and strong dispersion give them remarkable brilliance and fire. Diamonds form under high-pressure, high-temperature conditions deep within the Earth's mantle and are brought to the surface through volcanic kimberlite and lamproite pipes. Major diamond sources include Russia, Botswana, Canada, Australia, and South Africa. While most diamonds are colorless to slightly yellow or brown, trace elements can create fancy colors such as blue (boron), yellow (nitrogen), or pink (structural defects). Diamonds are commonly faceted for jewelry, with the brilliant cut being the most popular to maximize light performance, and they are also used in industrial applications due to their extreme hardness.