Along with their revered standing, diamonds have a rare geological backstory in contrast to every other materials discovered on earth. A pervasive fable about how pure diamonds are fashioned is that they arrive from buried coal, squeezed at excessive pressures. However the actual story is much extra thrilling. Scientists are persevering with to unravel particulars about how they type and what they will inform us in regards to the internal workings of our planet.

Diamonds are unique as a result of they originate from so deep contained in the earth. The diamonds we discover have been transported near the earth’s floor by particular magmatic eruptions, forming the rocks often called kimberlite, and sometimes lamproite. This magma rises shortly from the mantle as much as the floor, generally whisking diamonds alongside for the experience as unintended passengers.

Most pure diamonds—say 98 p.c—come from a depth of about 150 to 200 kilometers, within the base of the thickest and oldest elements of continents. Some uncommon sorts of pure diamonds are fashioned at even better depths, estimated to be as deep as 800 kilometers and maybe extra. This info alone attracts an vital distinction round diamonds. Nearly all minerals—together with most gem stones—, we are able to entry or mine on the earth’s floor have been fashioned throughout the crust, at comparatively shallow depths lower than 30 to 50 kilometers.

Learn Extra: Why Pink Diamonds are a Miracle of Nature

This info has been pieced collectively by finding out small items of different minerals, equivalent to kyanite crystals, that are generally trapped inside diamonds as they crystallize. The trapped supplies, or inclusions, are a bodily snapshot of the birthplace of a pure diamond’s formation and can be utilized to decipher the geological circumstances of diamond development. Primarily based on the examine of diamonds and their inclusions, we all know that the majority diamonds develop from carbon-bearing fluids within the mantle as they migrate and stream by means of the cracks in rocks. Because the fluid percolates, adjustments in strain, temperature or composition could cause carbon to crystallize out of the fluid right into a strong type diamond. Whereas inclusions can provide perception into the processes of diamond formation, they will additionally allow us to constrain the age of a diamond.

So are you able to date again to when a specific pure diamond was fashioned? Sure! Courting is feasible as a result of naturally occurring radioactive isotopes in some inclusions act like an inner clock. This sort of evaluation is harmful and it’s only attainable with specific sorts of inclusions. However, analyses of diamonds formations from a number of deposits world wide have revealed the unbelievable outcome that many diamonds are greater than 1 billion years outdated. Measured ages vary from a whopping 3.5 billion years outdated to a mere 90 million years outdated (!). To assist put this in perspective, the Earth is 4.56 billion years outdated and the mass extinction that killed off the dinosaurs was 65 million years in the past. Evidently, pure diamonds are fashioned throughout a variety of geologic time.

Along with their deep mantle origin and million-to-billion-year ages, scientists have realized that pure diamond formation isn’t just an remoted, erratic prevalence. Fairly, how pure diamonds are fashioned seems to correspond with bigger scale geological phenomena. For instance, the earth’s floor is split into gigantic tectonic plates and when one in all these plates bends and sinks into the mantle in a course of referred to as subduction, it may well find yourself offering the elements to set off diamond development. Even at depths past 660 kilometers, which marks the highest boundary of the decrease mantle, some diamonds comprise elements that have been dragged down into the mantle by subducting plates. Blue boron-bearing (sort IIb) diamonds have been argued to comprise boron and traces of water derived from deeply subducted tectonic plates, as illustrated under. Ongoing analysis on diamonds like these is shedding gentle on large-scale processes, such because the deepest reaches of the water and carbon cycles, and the way they behave over billions of years.

For additional background studying on diamond formation, see the G&G article “Current Advances in Understanding the Geology of Diamonds.”