This story was updated at 5 p.m. E.T.
A 100-carat, nearly flawless diamond is slated to hit the auction block today at Sotheby's in New York City.
The crystal-clear gemstone could fetch up to $25 million, the auction house said. The gorgeous jewel, which is the largest emerald-cut, flawless diamond to be auctioned off, is the centerpiece of the Sotheby's Magnificent Jewels auction.
Diamonds may seem almost otherworldly in their beauty, but the sparkling gemstones are actually made of one of the most quotidian elements — carbon. However, unlike in hunks of coal, a diamond's carbon atoms are organized into a highly structured lattice. Diamonds also exhibit something called maximal symmetry, meaning the bonds among the gems' carbon atoms can't be crushed or shifted to produce a more symmetrical shape. This is partly why diamonds look the same from any edge.
These gemstones are also relics of an ancient Earth. Though geologists are still working out the details, most believe that diamonds were formed in the depths of the Earth sometime between 1 billion and 3 billion years ago. [Sinister Sparkle: Tales of 13 Mysterious & Cursed Gems]
"You're talking on the order of 100 kilometers (62 miles) or more down into the Earth," George Harlow, a geologist who specializes in mineralogy and crystallography at the American Museum of Natural History in New York City, previously told Live Science.
At that time, buried carbon dioxide was heated to about 2,200 degrees Fahrenheit (1,200 degrees Celsius), then compressed with a mind-crushing pressure of 725,000 pounds per square inch (4.93 million kilopascals), according to a 2012 study in the journal Nature.
After enduring this subterranean pressure cooker, the diamonds likely took an express route up to the surface via a plume of mineral-rich magma known as a kimberlite. These plumes travel at an average speed of 22 to 25 mph (35 to 40 km/h). During volcanic eruptions, debris from these magma plumes can reach twice the speed of sound, Harlow told previously Live Science.
"If you were there, you would see the most impressive explosion, then immediately be dead because of the shock wave," Harlow previously told Live Science.
The diamond on auction is flawless and colorless, meaning it has no visible blemishes or inclusions, or faults, even when viewed under tenfold magnification, according to the Gemological Institute of America (GIA).
Diamonds typically acquire faults during formation when little bits of other minerals found in the Earth's mantle, such as olivine or garnet, get into the mineral, Harlow told Live Science. Other flaws come from an alternate form of carbon. Diamonds get their color from small amounts of impurities such as nitrogen, which get swapped with carbon atoms in the crystal lattice, Harlow said.
A flawless diamond, by contrast, is made of pure carbon. To be flawless, a diamond's crystal structure must also be perfect, with no tiny cracks, or mistakes, in the crystal structure, Harlow said.
"Some of those [cracks] are due to the fact that most diamonds are very old and they've been sitting in the mantle for a long time," Harlow said. "Even though we think we live on a solid planet, the solids are moving over time, and the diamonds get deformed, they get kind of crunched."
Most flawless diamonds were actually "perfect" sections cut from much larger hunks of rock, Harlow said. And large, 100-carat diamonds aren't too common to begin with, he added. So the new diamond is especially unusual.
"We're talking rare upon rare," Harlow said. Geologists still don't know exactly how flawless diamonds form, he added.
Though the current diamond would make one gigantic engagement ring, the gem isn't anywhere close to the biggest diamond ever found. That would be the "Cullinan I" or "Star of Africa I" diamond, which clocks in at 530 carats and is mounted in the British sovereign's royal scepter. The Star of Africa I was originally cut from a 3,106-carat monster that was unearthed in Premier Mine in Pretoria, South Africa, in 1905.
But size isn't everything. The spectacular blue Hope Diamond, which is estimated to be worth at least 250 million dollars, weighs just 45 carats.
Editor's Note: This story was edited to include additional information about flawless diamonds from George Harlow.