Unlike both the Constellation and the Lesedi, however, it is covered in carbon (at the moment it looks like a big lump of coal), which makes exactly what kind of diamond material is inside a “mystery,” according to Ulrika D’Haenens-Johansson, a senior research scientist at the Gemological Institute of America.

It also makes “the risk that much greater,” Mr. Pruwer said. When the stone was unearthed, there was a fair amount of speculation that it may be worth significantly less than its not-quite-as-giant siblings.

The profitability of any large stone depends on its yield: how many gem-quality carats can be gotten out of it once cut to maximize the price, which is in turn a function of the impurities in the stone — though, as Ms. D’Haenens-Johansson points out, even the impurities have value in a stone this size. They can reveal when the diamond was created and at what depth in the earth.

The mine, which has examined the diamond through a tiny “window” in the dark covering and scanned it with lasers, describes the stone as “near gem quality,” with “domains of high-quality white gem.” There are thousands of gradations of diamonds, ranging from D-flawless (the most rare) to industrial stones used in cutting and manufacturing.

“Is it D or D-flawless, and how big is the flawless part? I don’t know,” Mr. Burke said, acknowledging that the purchase “took a little bit of guts and trust in our expertise.” (To be fair, LVMH can afford it; its revenues in 2018 were 46.8 billion euros, or $52 billion.)

Still, Mr. Post said, “You don’t buy a stone like that unless you have some plan for what you are going to do with it and some belief that there is enough clear material that you can cut it and make a profit.”