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You know the cliché. But it’s not just diamonds that are forever. So too, apparently, are their press previews.

Well, forever is a stretch. But it’s hard not to count the seconds and glance nervously toward the door when the door is locked, guarded, and made of austere metal. As far as I could tell, curators and press officers did little to preclude the attendance of international jewel thieves at this morning’s viewing of the Wittelsbach-Graff blue diamond, a storied gem that the National Museum of Natural History is displaying alongside the Hope Diamond through August. But if there were any would-be Phantoms in attendance, they hadn’t a hope of escaping with the luminous, deep-blue goods. Once guards slid shut the doors of the Harry Winston Gallery, they didn’t open them again until the Wittelsbach-Graff was back in its vault.

Some 400 years after its likely removal from the Kollur mine in India’s Deccan plateau, the Wittelsbach-Graff still arouses intense interest. It sold to the jet-setting British diamond dealer Laurence Graff in 2008 for around $24.3 million after spending half of the previous century in unknown custody. Graff, on hand at the press event, said the value has only increased since—-because of the market for rare gems, and because he had the rock polished and recut not longer after he bought it. Flawless, it weighs in at 31.06 carats.

For Graff, the joy of the Wittelsbach-Graff is almost entirely in beholding it: “The main thing for me is not the history,” the well-tailored East Londoner said in a refined, ambiguously continental accent to a crowd of about 20 journalists and videographers. Toward the end of the event, Graff balanced the stone atop the back of his fingers while his lips curled slightly into a sfumato-esque grin. “It is an incredible feeling to be holding the world’s most incredible diamond,” he said. “Amazing.”

But the Wittelsbach—-large for a blue diamond but underwhelmingly small on first glance—-is inseparable from its history: It is likely the same blue diamond that King Philip IV of Spain gave to his relative Leopold I of Austria to marry his daughter, the Infanta Margarita Teresa, in 1664. It again became a dowry in 1722, when it passed to the Wittelsbachs of Bavaria, part of the Germanic region’s ruling family. It stayed there until 1931, when it was slated to sell at a Christie’s auction. But it disappeared under still-mysterious circumstances, resurfacing again for an exhibition in Belgium in 1951. In the early 1960s it sold to a private collector.

At the press preview, I mostly settled for observing the diamond through other attendees’ iPhone screens as they craned to take snapshots. Jeffrey Post, the curator of the National Gem Collection, explained that like the Hope Diamond, the Wittelsbach-Graff luminesces orange under ultraviolet light, although there was no such demonstration. Mostly the diamond begged an obvious question: How could a thing so small have witnessed Europe’s age of emperors, its great wars, its intrigues, and its romances?

I pondered that question for a while, and then I pondered why my watch says it’s Wednesday when, clearly, it’s Thursday, and then I meditated on where I should grab a snack before heading to the office. And then I paced around the center of the room a few times. And then finally, with the slide of a metal door, the museum released its captive audience.

But visitors to the exhibit, which opens tomorrow, are in luck: Because the diamond will be displayed behind glass and not in the open, they can leave the gallery as soon as they want.