The hardest natural substance is the diamond. Kimberlite is a form of igneous rock that contains diamonds. The diamond itself is just a crystallised chain of carbon atoms. The tightly packed structure of the carbon chains gives the stone its remarkable toughness. Kimberlite, like other igneous rocks, was created over thousands of years by volcanic activity that took place during the development of the earth’s crust. Inside these ancient spheres of volcanic activity—often near mountain ranges—Kimberlite is found in vertical shafts that stretch deep into the earth. Diamonds, one of several minerals found inside the Kimberlite, are found in sporadic deposits. Not all Kimberlite, however, includes diamond. Mica, garnet, and zircon are some of the other stones that are frequently found with diamonds. Kimberlite can be blue-grey in colour (known as blue ground) or yellowish in colour (known as yellow ground) when exposed to air.