Around 1920, Justo Daza, an experienced mine worker, and Fritz Klein, a mining engineer, were scrambling over the steep mountainside terraces of Chivor, a legendary emerald site in northeastern Colombia. They were breaking rocks with long iron poles and explosives packed into drill holes. They were hunting for new emerald veins but not finding any.

Daza insisted they keep trying in the area, so they upped the dose of explosives and blasted open a gaping hole that revealed promising glints of a mineral vein. Klein thrust his arm into the hole and fished out bits of quartz, feldspar and apatite — a phosphate mineral like that found in bones and teeth.

He probed deeper, until his hand closed around something big, dense, faceted and thrilling. Without even looking, Klein knew he’d struck green.

The prospectors had unearthed what would come to be called the Patricia Emerald: a dazzling 12-sided crystal roughly the size of a soup can, with a weight of 632 carats — more than a quarter of a pound — and a verdant color so pure and vivid you'd swear the stone was photosynthesizing.

Klein sold the find for tens of thousands of dollars, while Daza, predictably enough, "was given $10 and a mule," said Terri Ottaway, museum curator at the Gemological Institute of America.

Yet the public arguably got the best deal: The stone was later donated to the American Museum of Natural History in New York. Today, it remains one of the largest uncut emeralds in the world, and it will be a featured star when the overhaul of the museum’s gem and mineral halls is finished in 2019.

In its raw, columnar beauty, the Patricia encapsulates an often-overlooked feature of gemstones, especially the ones we deem "precious": diamonds, rubies, sapphires and emeralds. We might covet the stones for personal adornment and status flashing, but their real power lies in what they reveal about the dynamo that forged them: Planet Earth.

For scientists, a gemstone is a message in a bottle. Except the message is the bottle, a glittering clue to the extreme physical, chemical and tectonic forces at work deep underground.

Moreover, many of the qualities that helped loft the Big Four to prominence — their exceptional hardness, the depth and brilliance of their coloring, their rarity — are also key to the jewels’ scientific value.

Precious gems are born of strife, of shotgun marriages between hostile chemical elements, and they’re tough enough to survive cataclysms that obliterate everything around them.

The rules of gem science are not cast in stone. Researchers lately have been astonished to discover that some of the world’s largest and most valuable diamonds, which can sell for hundreds of millions of dollars, originated 250 miles or more below the surface, twice the depths previously estimated for Earth’s diamond nurseries.

Some diamonds turn out to be surprisingly youthful, a billion years old rather than the average diamond’s 2 billion to 3 billion. Other researchers have linked ruby creation to collisions between continental landmasses and propose that the red jewels be called "plate tectonic gemstones."

A team at the University of British Columbia analyzed newly discovered sapphire deposits in Canada’s Nunavut territory and concluded the stones there were generated by a novel three-part geochemical "recipe" unlike any described for sapphire formation elsewhere in the world.

You start with limestone sediments containing just the right mineral impurities — nepheline is a must! — and you squeeze and heat the rocky mass to 1,472 degrees. You add fluid and allow to cool. Finally, just when the growing mineral assemblage shows signs of instability, you inject another shot of fluid and lock the crystal into place. Total cooking time: about 1.75 billion years.

"If one step is left out," said Philippe Belley, a geologist at the University of British Columbia, "you don’t get sapphires."

In the past, geologists often dismissed gemstones as baubles and gem science as oxymoronic. "Gems were considered crass commercial materials and beneath the dignity of an academic," said George Harlow, curator of earth and planetary sciences at the American Museum of Natural History.

More recently, geologists have seen the refracted light. "My colleagues know that a gem course done as an introductory part of an undergraduate education is a really good hook," Harlow said. "When you can show how gems form, or the properties they have, it takes a lot of chemistry and physics to understand that."

Persuading large numbers of carbon atoms to lock limbs in all directions requires Stygian whips of high heat and pressure, as until recently could be found only underground. In theory, the Earth’s mantle, which is thought to hold about 90 percent of the planet’s carbon supply, is practically glittering with diamonds at various stages of formation.

Getting those jewels to the surface in bling-worthy condition is another matter. Diamonds must be shot up from below quickly — say, through a volcanic eruption — or they’ll end up as so much coal in your stocking. Researchers have discovered diamonds that had blundered crustward slowly enough for their carbon bonds to expand, leaving a stone with the shape of a diamond but the consistency of graphite.

Gareth R. Davies, a professor of geology at Vrije Universiteit Amsterdam, and his colleagues have recapitulated the reversion process in the laboratory. "Yes, we get diamonds and turn them to graphite for research," he said. "And my wife wonders why I’m such an idiot."

Researchers can also fabricate diamonds in the laboratory, although the results are more often destined for industry than Tiffany.

Nor can scientists create anything remotely as celestial as the Hope Diamond, the world’s largest deep-blue diamond. Researchers have plied the 45-carat diamond with every noninvasive tool in their arsenal, seeking to understand the precise distribution of boron atoms that lend the Hope its steely blue tint and why the diamond will glow, or phosphoresce, a spectral shade of blood orange when exposed to ultraviolet light. The phosphorescence could be the result of interactions between boron and nitrogen impurities in the diamond’s near-flawless carbon frame.