Spinel has a chemical formula of MgAl2O4 – one atom of magnesium to two atoms of aluminum and four atoms of oxygen. It also has an isometric crystal system. That means its crystals have axes of equal sides and they intersect at 90° angles. (This system is also known as the cubic system). Minerals with isometric crystal systems can occur with octahedral shapes, like this cranberry-red spinel from Luc Yen, Yenbai Province, Vietnam. © Rob Lavinsky, www.iRocks.com. Used with permission.