Diamonds are quite popular and very expensive in times. Reasons being, they are rare, and hard to come by.. They are only created by carbon being compressed by extreme heat and pressure, being in volcanoes, dormant volcanoes, or once been volcanoes.
The reason being of diamonds being so hard and beautiful, is due to the compression of many molecules of the very common carbon, into a very small space. Carbon can be found in EVERY organic material ever created. Since carbon has so many free bonds, they can link at four places with each molecule. Carbon can be linked infinitely, and the covalent bonds hold very strong and are quite stable, coupled with pressure and heat.. this creates a very strong diamond lattice structure that is very hard to break. Nitrogen and boron can be introduced during creation, slightly instilling a color impurity (yellow or brown for nitrogen, and blue for boron). If a diamond does not have enough heat and/or pressure, it will simply turn into graphite - regular pencil lead. Think of your penci lead as being a failed diamond, not quite reaching a certain temp for conversion into this "superstructure". We can also create synthetic diamonds through a man-made process recreating the conditions of heat and pressure, therefore creating any type of diamond we please. The most abundant and stable form of carbon, Carbon-12 does not decay due to it's natural stability, therefore remaining a molecule perhaps forever.
The reason diamonds are so nice to look at, is it's angle of refraction of light. It tends to reflect little light and disperse the rest of it at very acute angles. It may act as a sort of "prism" slightly scattering the colors of light(therefore us seeing it's colorful sparkle".
So really it's a miracle recipe to create diamonds formed into perfect octahedral structures, but from a simple and very abundant(the most abundant atom). Take regular things, throw in a little heat, pressure, and magic, and we create a stone so hard and rare we call a diamond. So think the next time of buying a diamond; it could have been simple common graphite..