A history of a planet’s volcanic activity is contained in its rocks. And Mars’ rocks - or at least many of the meteorites that have made the journey from it to Earth - were telling a contradictory story. They appeared to be highly oxidized, but our understanding of the internal volcanic forces on Mars meant that they should have been formed in a reducing environment - the exact opposite of what the chemical makeup of the rocks themselves indicate. Now, a new paper published in the Journal of Geophysical Research: Solid Earth from planetary scientists Hideharu Kuwahara and Ryoichi Nakada of Ehime University provides an answer as to why - and it involves a gemstone.

That’s not to say it’d be the type of gemstone that a jeweler would set in a ring. But majorite, a type of garnet that is only formed under massive pressures and temperatures deep within the mantle of planets, appears to be the reason for the contradictory rock signals.…