The world’s oldest time capsules have been opened, and they’re full of treasure 3.5 billion years old. These time capsules are diamonds (nothing to sneeze at themselves, when it comes to treasure), and the passengers they’ve carried down through the eons are tiny mineral grains containing sulfides, chemical compounds composed of sulfur and oxygen. The ages of these sulfide grains can be precisely dated using a method that measures the relative abundance of rhenium and osmium, two radioactive elements that are often concentrated in sulfide-bearing minerals; rhenium breaks down into osmium at a well-known, easily calculated rate. According to recent calculations, the diamonds from Slave Craton, a rock formation in Canada’s Northwest Territories, are the oldest known diamonds in the world, slightly older than their Siberian and African counterparts.

In many ways, diamonds are as critical to our understanding of the history of the Earth as they are to crafting fine jewelry. As the hardest and most durable material in nature, diamonds provide the ideal matrix for storing primordial materials like the aforementioned sulfide crystals, which were accidentally included in the growing diamonds as they were formed deep below the surface of the Earth.

The Canadian diamonds apparently started out as carbon-rich fluids locked up in rocks on the Earth’s surface. So how did that carbon get so far below the Earth’s surface that it was converted into diamonds? As it happens, the continents are made up of massive plates of relatively light rock that floats on top of the mantle, a semi-liquid layer of denser material dozens of miles down. These so-called tectonic plates butt up against each other, causing earthquakes and volcanoes, and eventually one will ride down under its neighbor, to be recycled in the mantle. As a result, surface material can easily end up over a hundred miles below the surface. During eons of geological time, the carbon-bearing rocks are dried out and subjected to such crushing pressures that their carbon rearranges itself into diamond crystals of various sizes. Eventually, some diamonds are included in magma that’s spat back out onto the Earth’s surface, where they’re collected by humans and fashioned into diamond rings for discerning consumers like you. The oldest such diamonds are found in cratons, ancient segments of continental rocks that are rooted deep in the mantle, and around which younger continental rocks grow.

Along the way, some diamonds pick up inclusions of other materials, tiny specks that may end up buried deep within the crystal. To jewelers, they’re imperfections to be scorned; but to scientists, they’re miniature treasure troves. The diamonds they’re imbedded in may never be set into engagement rings, but they do offer rare glimpses deep into the Earth’s past. Recently, such flaws were removed from a series of small diamonds mined at Slave Craton in Canada. Subjected to rhenium-osmium dating, the sulfide-bearing flaws were found to be 3.5 billion years old, having been formed during the geological period known as the Archaen Era (2.5-3.9 billion years ago). So not only do cratons contain the oldest rocks in the world, they offer the oldest jewels too. It’s up to us to find them and make them into eye-catching diamond jewelry — or handy drill-bits, as the case may be.

To put their age into perspective, consider the fact that the earliest known forms of life (tiny one-celled bugs) seemed to have appeared sometime late in the Archaen. Next time you’re shopping for jewelry, and you find yourself eying all those diamond rings, consider the fact that the stone in the engagement ring you’ve got your eye on may well be older than the entire history life on Earth. If that doesn’t blow your mind, nothing will.