Diamonds are one of the hardest materials known to science, making them not only beautiful, but also useful for industrial purposes. They are formed in extreme conditions of heat and pressure from carbon atoms.
Coal and diamond creation
Some students, it seems, are still taught that coal is involved in the formation of most diamonds mined on Earth, but this is highly unusual. Most of the diamonds ever dated proved to be at least 100 million years older than the earliest known plant life (required to form seams of coal). Diamonds in general were formed at depths in the Earth to which coal has never been introduced.
Diamond crystals form under conditions of very high heat and pressures that would easily crush any metal made by humankind’s science. They are relatively rare because such conditions “… occur in limited zones of Earth's mantle about 90 miles (150 kilometers) below the surface where temperatures are at least 2000 degrees Fahrenheit (1050 degrees Celsius).” This is “unusual” because diamonds form ideally under the center of a continental plate, in the upper mantle of the Earth.
Diamond must have been, for humans to have found them and mined them commercially, expelled from their deep-set original formations. Geologists hold that ancient volcanoes of extraordinary power accomplished this transportation. These volcanoes, none of which has been seen in recent memory, were sufficiently violent to originate in the mantle and to bring up pieces of their overlying continental plates, where diamonds formed hundreds of millions – or billions - of years ago.
Due to the long-term effects (a matter of hours) of heat and pressure on finished diamonds, “if they were traveling too long and too slowly they would have literally turned into graphite along the way.” For this reason, scientists believe that the delivery of pykes, or tubes of diamonds, to the surface was not only violent, but that it occurred very quickly, allowing the diamonds present as inclusions to cool.
“Tiny diamonds have been found in rocks that are thought to have been subducted deep into the mantle by plate tectonic processes - then returned to the surface.” Subduction would allow the production of diamond crystals at lesser depths than usually required, and some of the carbonaceous materials placed together might have included remains of sea creatures, though not coal.
Not all formed in the Earth
Meteor impacts have been taking place around the world since it was created. They continue today, and the hypervelocity present at impact between meteoroids and the surface of the Earth is sufficient to form diamond crystals. “Coal could be present in the target area of these impacts and could serve as the carbon source of the diamonds.”
Diamonds are useful, beautiful, and rare enough to be valuable when men and women look for shiny jewels to use as gifts. Their production, whether earthly or in space, is uncommon enough that these crystals likely will retain their high value until science allows humankind to develop a super-material prettier and harder than the natural stones.