It’s time to dig deep in the mantle for blue minerals: Kyanite and Ringwoodite.

Kyanite is beloved for its beauty, unique characteristics, and distinct formation story. The deep blue is prized by collectors, and is an offbeat birthstone for September. It has variable hardness, softer (Mohs 4.5-5) along the length of the crystal and harder (Mohs 6.5-7) across the short dimension, making it an easy favourite of students during mineral identification exams. It’s also an indicator mineral, whispering to scientists about formation conditions as they unravel the story of a rock.

Ringwoodite is a blue mineral formed in the high pressure and temperature. It forms in the transition zone between Earth’s upper and lower mantle, and polymorphs into other minerals when brought to the surface unless trapped within the pressure chamber of a diamond’s heart. It was first discovered in a meteorite in 1969, and has been found in the shock-melt of other meteorites that have undergone extremely high temperatures and pressures. While extremely rare at the Earth’s surface, Ringwoodite is so common at depth that it may be responsible for containing enough broken-down water molecules to refill the oceans.

Voting for each match starts and ends at 06:00 GMT. Due to site limitations, the polls will continue to be visible until manually closed by volunteers, but only votes cast in the 24-hour period will be tallied.