Dr. Harlow suggested that precious gems gained their reputation in part by their association with gold. As insoluble stones, the gems ended up concentrated at the bottom of stream beds, often in the vicinity of similarly insoluble gold. Long prized for its ductility, beauty and resistance to oxidation, gold was considered the property of rulers and kings, so why not the glittering stones found beside it?

The word diamond stems from the Greek terms for “indestructible” and “that which cannot be tamed,” Dr. Harlow said, “and those attributed metaphysical properties made the ruler seem even more important.”

Diamonds are not indestructible, but they are the hardest substances known, given the top score of 10 on the Mohs scale of hardness — that is, resistance to scratching. Behind a diamond’s untameability is its three-dimensional structure, a repeating crystalline lattice of carbon atoms, each one strongly bonded to four neighbors atop, below and to either side.

(In graphite, by contrast, carbon atoms are bonded together only in two-dimensional sheets, which will flake apart with the simple act of putting pencil to paper.)

Persuading large numbers of carbon atoms to lock limbs in all directions requires Stygian whips of high heat and pressure, as until recently could only be found 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 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.”