Who wouldn’t love to understand the secrets of the earth that reveal where diamonds might be located? Before any entity sends down an exploratory earth-sampling device into the ground, someone did a lot of research on the probability of success in finding diamondiferous ore.
It might surprise you to learn that in 2021, there are only about 50 major active diamond mines worldwide. Because of the geological conditions where diamonds are found, the mines are located primarily in very remote, often very harsh environments.
If you’re wondering how sleuths first search for possible diamond-rich ground, think kimberlite – an igneous rock known to harbor diamonds if one digs deep enough. Kimberlite gets its name from the town of Kimberley, South Africa, where the discovery of a massive diamond weighing 83.5-carats, (Star of South Africa) in 1869 spawned a diamond rush there.
Kimberlites are carrot-shaped vertical structures occurring in the earth having been formed some 90 to 280 miles below its surface. Deep within the earth’s mantle in the area right around the kimberlite, are thought to be diamondiferous soil. At this depth, under conditions of extreme heat and immense pressure, diamonds generated from one element – carbon.
But explorers also look for other tell-tale signs that diamonds might be present in the targeted terrain. Kimberlite serves as a carrier for other minerals that are sampled in the extracted earth during exploration. Garnets, particularly rich purplish-red varieties, are indicators that diamonds are probably nearby. Gem geeks (like us) love to find that unusual diamond rough with an embedded garnet crystal from time to time. In this case – the garnet is not merely nearby – but actually inside the diamond!
But prospectors also look for peridotite, made of mostly olivine and pyroxene, and mantle, a silica rich material. Xenoliths (a rock material naturally encased in another igneous substance) is also sampled as an indicator in diamond exploration.
So, the kimberlite is a good place to start, but the sampling of these specific materials within the earth are strong indicators that geologists have reached pay dirt, literally.
The initial challenge for mining and exploration firms is to find the kimberlites, which usually occur in clusters. It’s not always so straightforward a discovery by a visual inspection though. While geologists have the means to predict where kimberlites are most likely located, they have usually become covered well below the earth. The exciting part happens when a kimberlite is found. It must be sampled to discover if it contains diamonds and indicator minerals – and if the quantity supports further development of a lucrative mine.
How much diamonds need to be recovered in kimberlite exploration to be favorable for further mining you ask? The actual diamond volume in a sampling of kimberlite is quite small relative to its mass of host rock. To put it in perspective, even the richest deposits in the world may yield just a gram of diamond per metric ton of rock; that’s 1 part per million. Along with that haul are much larger quantities of other minerals that hitch a ride with the diamonds. These minerals provide clues that point to the location of its source. However, even this can challenge experts when you consider that natural weathering, erosion, and glacial movements occurring over millions of years may have flung these minerals across vast distances from their origin.
Today we speak about jewelry’s storytelling potential and it’s certainly valuable in creating consumer loyalty to certain gemstones. But before that story – there was the rich tale of diamonds discovery and the sampling of other minerals that foretold where the mother lode might be found.