Good diamonds are worth a fortune, a girl's best friend, one of the most common engagement ring stones, the birthstone for the month of April, and so on. These commonly-known diamond tidbits only scratch the surface of the hardest stone known to man. There is much, much more to diamonds than meets the eye, although a diamond's unique fire and clarity is most of its price.
|HULK DIAMOND SMASH!|
Besides the usual clear color, diamonds can come in any hue of the rainbow. There are even black diamonds like the one above. All sorts of lore accompanies diamonds of different colors; for example, black "carbonado" diamonds may have come from space, and the Great Hope Diamond is rumored to be cursed.
The Great Hope Diamond in particular illustrates a relatively-unknown property of diamonds: They glow in blacklight. In particular, the Hope Diamond, which looks blue in normal light, turns red-orange when placed in ultraviolet light. The exact color depends on trace elements within the diamond. Given that diamonds are usually thought of as a clear stone, it was quite a shocker to see a whole butterfly of multicolored diamonds in the Field Museum! (No, you cannot go see it- the exhibit was temporary.) This property can be used to determine whether a diamond is real or not; a real diamond should look like hippie gear under blacklight.
|Or an eye, staring into your soul and waiting to curse you.|
The truth is, however, that diamonds are carbon. Just carbon (trace amounts of other elements not withstanding). Carbon is an extremely common element that forms the basis for all life on Earth (thus "carbon-based lifeforms"). Other examples of pure carbon include charcoal and graphite, AKA pencil 'lead.' Notice how none of the nouns except "diamond" in this paragraph fall under the category of "hardest thing around."
Basically, it depends on how the carbon atoms are arranged. The crystal structure of a diamond is what provides its literally adamantine hardness, luster, 'fire,' thermal conductivity, and other unique properties. To form this unique crystal requires high pressure and relatively low temperatures; otherwise, the diamond will instead become another sort of carbon. There are ways to transform diamond into graphite, but unless you happen to be an alchemist and are able to reverse it quickly, this rather extreme process (involving a vacuum and temperatures above 1,700 degrees C) is not recommended.
You must be more badass than this to turn diamonds into graphite. Seriously, that's just a dick move.