|3D movie: link|
Why is diamond so tough, so adamant, so opposed to physical change? I think the answer is called "perfect covalency" but not in an electron sharing sense:
The paradox of the diamond is interesting. Its atoms are not arranged in a tight, closest-packing order. They lack the triangulation of sound architecture. In order for its remarkable rigidity to be understood, I assume that the electrons which surround this meager structure supply it with its resistance to deformation. This is one reason why I cannot assume that electron clouds can infiltrate one another like vapors or ghosts. linkPure diamond is also colorless and transparent, so what gives fancy (colored) diamonds their colors? The answer is not simply: "there must be something blue inside." An impurity is involved, but not the usual colored metal atoms like iron or chromium found inside other gemstones. The impurities in blue and yellow diamonds are carbon's left- and right-hand periodic neighbors--boron and nitrogen--playing little tricks on the lattice electrons.
Take the perfect 3D lattice of carbon atoms pictured above. Now suppose that we could randomly go in and replace every millionth carbon atom with a boron atom without perturbing anything else. What we get is a boron-doped diamond lattice. Because boron has one less electron than carbon, the entire lattice structure of the diamond is riddled with electronic "holes."
Now it just so happens that reddish-orange light has just the right energy match to promote an electron on an adjacent carbon atom into a "hole" next door. That jump in turn creates a new "hole" and so the next neighbor carbon jumps at the chance to fill the new hole and so on and so forth throughout the entire diamond lattice. Really, a blue diamond is rather like a doped silicon p-type semi conductor. In fact, blue diamonds are semi-conductors--albeit rather expensive ones!
Because only reddish-orange light is absorbed, the remaining visible light appears bluish to our eye because the white light lacks its reddish-orange component: remember the color wheel and complimentary colors!
So what makes yellow diamonds? The answer is slightly more complex. If nitrogen atoms, carbon's other nearest neighbor, are doped into the diamond lattice instead of boron, each nitrogen brings an extra electron into the lattice which is easily promoted to the existing conduction band of diamond by violet light--ergo yellow appearing diamonds. Yellow diamond is analogous to an n-type semiconductor. Here's a link explaining in more detail why blue diamonds are blue and why yellow diamonds are yellow: Link.
Meanwhile, here's a very pretty picture of the Hope Diamond:
*I don't mean "blue diamond-shaped" Viagra either.