"Hematite is in rocks, too, and is also in much of the clay that is used to make bricks. Iron in the hematite (or other iron-containing minerals) rusts when exposed to oxygen and water." ...

In their paper, Jensen and Ault describe the use ... tiny hematite crystals that replaced the original magnetite crystal." Using martite samples obtained from a 1.7-billion-year-old rock situated ...