Diamonds are a girl's best friend OR How carbon turns into a diamond

In honor of my best pal Kathleen (who is an astrophysicist, just in case you were wondering) getting engaged, I'm going to discuss what makes a diamond a diamond (not not just a pile of carbon).

Carbon

Carbon atoms have four valence electrons in their outer shell. This means that they form four bonds to other atoms, such as oxygen, nitrogen, and hydrogen. These bonds are stable, but can be easy to break and make under certain conditions. This is a big part of why life is carbon-based rather than silicon based (it also has four valence electrons, but forms much stronger bonds to oxygen than does carbon).

Molecules made entirely of carbon exist in several forms (carbon atoms don't just hang out by themselves, after all). The form of molecular carbon I think of first is coal (the kind we burn as fuel). Coal consists of carbon atoms that are haphazardly bonded to other carbon atoms (and to other atoms like hydrogen and oxygen). Because of this, coal tends to exist as amorphous (shapeless) solids.

Graphite (pencil lead) is made almost entirely of carbon. It consists of sheets of carbon atoms that are bonded to three other carbons in six-membered rings. The geometry around each carbon atom is trigonal planar, meaning that the three bonds are 120 degrees apart- as far as they can get- and an overhead view would look sort of like a triangle. Because there are only three atoms attached to each carbon, this means that one of the bonds is a double bond; this leads to the semiconducting properties of graphite (bonds are made of electrons, double bonds that are close to each other allow electrons to travel, creating currents). Because the graphite sheets are not connected to one another, they can slide over each other (this is why graphite makes good pencil lead).

Diamonds are also made almost entirely of carbon atoms, but in this case the carbon atoms each have four carbon atoms attached to them (in a white diamond). The atoms are bonded in a tetrahedral arrangement, so that the carbons are as far apart from one another as possible. This structure extends through entire crystals, and is called the crystal lattice.

You might be wondering why graphite is fairly common, while diamonds are relatively rare. It turns out that graphite (because of the double bonds) is the more stable form of pure carbon. It would take more energy for diamonds to form than for graphite to form. One way diamonds are made is through the heating and compression of coal (this can happen if a meteorite hits the right place on Earth). Lots of energy (in the form of heat and pressure) goes in, forcing the bonds in the coal to rearrange- atoms other than carbon are lost, and bonds are formed to other carbon atoms as the rock cools, which sometimes results in the formation of diamonds.

Diamonds can be uncolored or colored. Colored diamonds occur because of impurities in the diamonds (they are not pure carbon, but have tiny amounts of other elements in their crystal lattice). Yellowish diamonds can occur when nitrogen is part of the lattice. Hydrogen can lend a bluish tint to a diamond, while pink diamonds are the result of a shift in the crystal lattice of a diamond. The shininess of diamonds is due to the crystal lattice, and their clarity to how many "breaks" there are in the lattice of that particular diamond crystal.

Diamonds can be mined or made in a lab (by applying pressure and heat to coal, under carefully controlled conditions). I would argue that the fake uncolored diamonds (lab made, called fiamonds by Kathleen) are of just as high quality (or higher quality) than ones that are mined. They would likely have fewer imperfections and cost less than diamonds that are mined.