Crystalized carbon. Diamond is a cubic mineral, like gold, spinel, fluorite, etc. Its ideal crystal shape is eight-sided -- two pyramids attached to each other. Its refractive index is 2.42, as opposed to cubic zirconia which is 2.15; sapphire and ruby which are 1.77; emerald, aquamarine, heliodore and morganite which are 1.57, and so on.
Diamond is rated as a 10 on the Mohs hardness scale, but only because it is the only natural gem harder than corundum (sapphire and ruby) which are a 9 on the scale. In reality, diamond is more like a 41 -- its not just a little harder than corundum, but mucher harder.
Diamonds are mined in South Africa, Australia, Siberia, Canada, Brazil and a few other lesser places. Most gem-quality diamonds mined are a light yellow or light brown color, but the stones occur naturally in every color of the rainbow, including colorless (which is the most popular for jewelry), milky translucent white and opaque black.
The Dresden Green, medium green, 41 cts.
The Tiffany Yellow, yellow, 128 cts.
The Wittelsbach, medium blue, 35 cts.
Jennifer Lopez's pink diamond, 6 cts.
The Amsterdam, black, 33 cts.
The Hope, dark blue, 45 cts.
The Allnatt, golden yellow, 101 cts.
The Star of South Africa, colorless, 47 cts.
The Pumpkin, orange, 5 cts.
The Conde, pink, 9 carats.
The De Beers, light yellow, 234 cts.
The Incomparable, brownish-yellow, 407 cts.
The Copengagen, blue, 51 cts.
The Polar Star, colorless, 41 cts.
The Victoria-Transvaal, brownish-yellow, 67 cts.
The Millennium Star, colorless, 203 cts.
etc etc etc