Diamonds are just carbon. Colorless diamonds are generally pure carbon, but diamonds of various exotic shades are almost always colored by tiny impurities within the structure of the stone. Graphite, the “lead” in pencils, is pure carbon too. The only difference between pencil lead and diamond is the way the molecules are bonded together. Graphite is a soft form of carbon that easily rubs off onto paper, while diamond is the hardest naturally occurring substance known.
The diamond’s tight cubic molecular structure is what makes it so hard. It forms under conditions of temperature and pressure that are almost inconceivable. Created deep underground, it forms at depths of 90 to 120 miles or more. The pressure upon it is about 45 to 60 kilobars at these depths. The temperature at which diamonds form is considered relatively low, however, considering this great depth. It is about 1652 to 2372 degrees Fahrenheit.
Scientists believe such conditions only exist in two places (other than modern laboratories). One is at the lithosphere mantle below ancient continental plates. The mantle under the oceans would be too hot, and higher in the earth’s crust would not produce enough pressure. The only other place on earth diamonds are created by nature, although most of them are microscopic, is at sites where meteors strike the ground.
The diamond that surfaces has become incorporated in a rock matrix called kimberlite, or sometimes in lamprolite. This large carrot-shaped mass of rock then blasts to the surface in a volcanic event, bringing up diamonds from abyssal depths to levels where they may be mined, or may even weather out of the rock and be found lying in gravel or soil. This process takes time. Natural diamonds are 1 billion to 3.3 billion years old, 22% to 73% of the age of the earth.
The commonest impurity in diamond is nitrogen. Nitrogen is an element that naturally tends to form molecules consisting of two atoms. If paired nitrogen atoms occur in a diamond, there will be no effect upon its color. If large groups of nitrogen atoms occur, the diamond may be yellowish or brown. But if single atoms of nitrogen occur throughout a diamond, the result may be a bright valuable Canary diamond. Perhaps 0.1% percent of gem diamonds are colored with nitrogen.
Boron is another element that can color diamonds. Diamonds colored with boron will show various shades of gray, blue, or slate. Hydrogen may also color diamond, tinting it different blues or sometimes violet. Unbelievably, hydrogen impurities may also create chameleon diamonds, whose colors may change from shades of green to shades of yellow or chartreuse under certain conditions of light or heat.
Green diamonds are created a different way, through (natural or synthetic) radiation. To understand how this works, it is necessary to remember that light is a form of energy. When light strikes a pure carbon stone, its energy cannot make the atom’s electrons move, because they are so tightly bound. None of the light’s energy is absorbed, used, in moving the electrons within the stone. Since none of it is used, it all reflects out, and this combination of all the colors of light lets us see a classic white stone. White light is a combination of all the colors of the rainbow.
But radiation bombarding a diamond over eons can displace some electrons in the lattice of atoms that make up the stone, creating “holes”. When light hits such a stone, these electrons will absorb some light energy, and move. When this happens to a green or blue-green diamond, the “loose” electrons absorb all the colors but green, and the green is reflected out. We see a green stone.
Shades of pink and red in diamonds are not caused by impurities so much as by lattice distortions caused during the creation of the diamond. The same kinds of distortions may also produce purple stones. Orange diamonds, which are extremely rare, are probably caused by nitrogen, just as yellow diamonds are, but under conditions of radiation and/or heat that alter the placement of nitrogen atoms in the stone.
All of the colored diamonds are essentially pure carbon. It is only the tiniest amounts of impurities, or the slightest bends in their structure that affect their optical properties and show us diamond colors. The hardness and adamantine luster pressed into the stones over buried centuries remains undimmed.