Enlarge / A 0.03 carat blue diamond analyzed for this study. The visible black specks are minerals trapped inside the diamond. (credit: Evan M. Smith/© 2018 GIA)

Diamonds have a lot going on. Besides their sought-after clarity, extreme hardness, and entanglement in civil wars, diamonds tell unique geologic stories. Contrary to the convenient idea that they are born from a magical transformation of compressed coal, diamonds actually form deep within the Earth and blast to the surface in unusually rapid volcanic eruptions.

What a jeweler might call an “impurity” in a diamond, a geologist calls an “inclusion.” These bits of other minerals trapped inside can tell you where that diamond came from and what it was like down there. In a new study of rare blue diamonds (like the famous Hope Diamond), a team led by Evan Smith of the Gemological Institute of America discovered that they appear to form at exceptional depths—yet are tinted blue by boron from ancient oceans.

Feeling blue

Only about 0.02 percent of diamonds are blue, making them a rarity among rarities. They’ve been found around the world, though, in volcanic deposits as old as 1.2 billion years and as young as 90 million. The weird thing about them is that the boron that turns them blue is exceedingly rare down in the Earth’s mantle and much more common in the Earth’s crust.