It's been said that diamonds are forever — probably because "diamonds are billion-year-old mutant rocks exposed to many lifetimes of crushing pressures and scorching temperatures in Earth's deep mantle" doesn't have the same snappy ring to it.

Either way, it takes a long, long time for a chunk of carbon to crystallize into a sparkling diamond — so long, in fact, that scientists aren't positive how they're made. One popular theory maintains that many diamonds form when slabs of seabed (part of an oceanic plate) grind underneath continental plates at so-called tectonic subduction zones. During the process, the oceanic plate and all the minerals at the bottom of the sea plunge hundreds of miles into Earth's mantle, where they slowly crystallize under high temperatures and pressures tens of thousands of times greater than those on the surface. Eventually, these crystals mix in with volcanic magma called kimberlite and burst onto the planet's surface as diamonds.

Support for this theory can be found in the oceanic minerals that give blue stones — like the infamous (and possibly cursed) Hope diamond — their signature hue. However, these diamonds are among the deepest, rarest and most expensive on Earth, making them hard to study. Now, research published today (May 29) in the journal Science Advances provides fresh evidence for diamonds' oceanic origins. For the study, the researchers looked at the salty sediment deposits inside a much more common class of stone, known as fibrous diamonds. [In Photos: Ocean Hidden Beneath Earth's Surface]

Unlike most diamonds that end up in wedding paraphernalia, fibrous diamonds are clouded with little deposits of salt, potassium and other substances. They're less valuable to jewelers, but arguably more valuable to scientists looking to uncover their underground origins.

"There was a theory that the salts trapped inside diamonds came from marine seawater, but it couldn't be tested," Michael Förster, a professor at Macquarie University in Australia and lead author of the new study, said in a statement.

So, short of tracing the ancient origins of an actual diamond, Förster and his colleagues attempted to re-create in their lab the hyperhot, hyperpressurized reactions that occur when seafloor minerals subduct into Earth's mantle. The team placed marine sediment samples into a container with a mineral called peridotite, which is a volcanic rock widely present at depths where diamonds are thought to form; then, they exposed the mixture to a combination of intense heat and pressure conditions that mimicked those found in the mantle.

The researchers found that when the mixture was subjected to pressures of 4 to 6 gigapascals (40,000 to 60,000 times the average atmospheric pressure at sea level) and temperatures between 1,500 and 2,000 degrees Fahrenheit (800 to 1,100 degrees Celsius), salt crystals formed with nearly identical properties to those found in fibrous diamonds. In other words, when the old seabed slips into the deep crucible of the mantle, the colliding forces create the perfect conditions for diamond formation. (Gem diamonds, which are made of pure carbon and don’t include any sediment deposits, can also be created this way.)

"We knew that some sort of salty fluid must be around while the diamonds are growing, and now we have confirmed that marine sediment fits the bill," Förster said. He added that the same experiments also produced minerals that are key to the formation of kimberlite, on which diamonds typically hitch a ride to Earth's surface during volcanic eruptions.

So, diamonds may truly be bits of ancient oceanic history you can wear on your finger. And if these gems are too expensive for your taste, don't fret— you can still wear a piece of the planet's extreme past by slipping on a gold or platinum ring. According to a recent study in the journal Nature, trace amounts of the shiny minerals in those common types of jewelry probably originated with an epic neutron star collision that literally rained bling on our solar system 4.6 billion years ago.

Originally published on Live Science.