Christie’s just announced two natural blue diamonds heading to its Magnificent Jewels auction at Rockefeller Center on June 9th, and the story behind their value is almost as fascinating as the stones themselves.

The headliner is the Azure Blue diamond. The 31.62-carat fancy blue pear-shaped diamond that Christie’s describes as the largest fancy blue diamond ever offered at auction. Its pre-sale estimate sits at $6.5 to $8.5 million. Sharing that estimate, almost exactly, is a 5.04-carat fancy vivid blue marquise-cut diamond also appearing in the same sale. Same estimate. Six times the size difference. And that, right there, is the story.

The process is extraordinarily rare. Blue diamonds primarily fall within the Type IIb classification, which accounts for fewer than 0.1 percent of all natural diamonds. Many Type IIb stones even conduct electricity, which is a physical property that makes them behave unlike almost any other gem material on Earth.

Within the world of blue diamonds, color saturation drives value as powerfully as size. GIA grades blue diamond color across a range that includes Fancy Light, Fancy, Fancy Intense, Fancy Vivid, Fancy Deep, and Fancy Dark. Fancy Vivid represents the pinnacle, the most saturated, most striking blue a natural diamond can achieve, and the rarest to find in nature.

The Azure Blue diamond carries a Fancy Blue grade. Magnificent color, extraordinary size, and GIA-graded as potentially Internally Flawless. It’s a stone that any serious collector would stop everything to consider. The 5.04-carat marquise cut, however, carries a Fancy Vivid Blue grade. That single word — vivid — closes an enormous size gap.

A Fancy Vivid Blue of over five carats with VVS2 clarity, potentially Internally Flawless, and Type IIb classification is a stone of almost incomprehensible rarity. The difference in color grade between the two diamonds essentially offsets the difference in size, which is why Christie’s estimates them identically. Nature and the GIA grading scale have produced a perfect equilibrium between two completely different stones.

The Azure Blue diamond sits in a platinum ring featuring a hidden halo of natural pink diamonds. The setting complements its even, distinguished blue tone without competing with it. The 5.04-carat vivid blue marquise sits in a platinum ring with baguette side diamonds. Both settings let the diamonds do the talking.