What is a Diamond? This question had been unanswered till the end of the seventeenth century. Even the relationship to carbon was suspected.
In 1694, two Italian academicians had done an experiment on diamond in Florence. They set up a large burning glass focused on a small diamond and saw it “cracked, coruscate and finally disappear”, leaving a minute quantity of blue ash. But it was the French physicist Babinet, who proved beyond reasonable doublet that a diamond was carbon in exceptionally pure form.
One of Lavoisier’s most celebrated experiments was to place a diamond in a bell jar filled with oxygen which rested in a basin containing mercury. The rays of the sun were then focused on the diamond by means of a large burning glass. After the diamond had been consumed, the bell jar was found to contain great quantities of carbonic acid, indicating to Lavoisier that the diamond was composed of carbon. Later experiments by Humphry Davy in England prompted him to conclude that the diamond was composed of carbon and nothing else, a view that was first contradicted in 1841 by Dumas and Strass whose numerous experiments revealed minute traces of other elements notably nitrogen and aluminum. It is these slight impurities that deform the perfect crystalline structure of the ideal diamond and can have a marked effect on the physical properties of a stone.
It would be an exaggeration to say that not much more is known about the origin of diamonds today, more than two thousand years ago when they were believed to be the splinters of stars. Nevertheless, it is true that there is still no unanimity among geologists about exactly how and where diamond is formed. As late as the nineteenth century, theories about the origin of diamonds had been shaped by the fact that the stones had always been found on or very close to the surface, either in riverbeds or in the beds of the rivers that had dried up ages ago. Even as late as 1869 the Gentlemen’s Magazine of London reported that a “continental experimentalist” had declared that the intense cold of stellar space disassociated and crystallized carbon from “masses of meteoric nature coursing through space” and caused diamonds to fall from the sky. The editor went on to comment that “the location of diamonds upon the earth agrees much better with the hypothesis of a sky source than an earth source” and that “those cope specimens now attracting so much attention are found on the surface of the ground only it is of no use to dig for them”, still the “continental experimentalist” may well have a point. Diamonds have been found in meteor craters at Novo Urei in south-eastern Russia and at Canyon Diablo in Arizona; although most scientists believe they were created by the heat and pressure of impact and not carried to earth in the meteors.
It was not until the discovery of the “dry diggings” at Kimberley in 1870, coupled with the determination of the miners to excavate every inch of their dearly bought claims that it became clear that diamonds came from below and not from above. It was also clear that diamond was invariably associated with one particular type of rock and that this rock was only to be found in clearly delineated areas. Since the rock and the diamonds persisted at depth, it was soon suggested that these “pipes” were volcanic in nature and that diamonds had been formed out of carbon under intense heat and pressure deep in the bowels of the earth. They had been forced toward the surface when those long extinct volcanoes had erupted millions of years ago. This strange diamond bearing rock, soon to be called Kimberlite was assumed to be nothing more than solidified lava. But as mining progressed at Kimberley it was discovered that the pipes were not great volcanic funnels plunging into the earth’s core.
A great deal has since been learned from the making of synthetic diamonds and perhaps the most widely accepted current theory is that since diamond forms at pressures and temperatures between 0.5 million pounds per square inch, the formation must have taken place at depths of at least 120 miles, chemical studies pointed to the ultra-basic rock peridotite in its molten form as the most likely to have provided the right conditions for the creation of diamond from carbon. The molten of crystallization is assumed to have been long and slow and the theory goes that conditions remained stable for a considerable period as a result of the pressure of carbon dioxide gas below became too great, the balance was changed and the diamond bearing magma was driven explosively towards the surface. On the way, it picked up other rocks and minerals forming itself into the “geological plum pudding” that we now call Kimberlite, eventually erupting through the surface of the earth and solidifying.