Iron's Uncommon Impact Within Gems

In recent On the Rocks columns, I’ve written about the elements and valence electrons.

- Bob Jones

Readers commented favorably about them, so this column will describe just the active metal element iron and its effects of color in minerals. We are all familiar with the iron minerals hematite, magnetite, siderite, and certainly rust. But we are not as familiar with the role iron plays in the color of gem minerals. Iron gives some of our favorite minerals very attractive colors. Without a trace of this common metal in the molecular structure of many minerals, they would really lack eye appeal.

Take benitoite, for instance. It is a rare and popular blue gemstone found in California. When first found, it was thought to be a blue diamond. It is so beautiful it has been named California’s official state gem.

Benitoite’s chemical composition is barium, titanium, silicate. By themselves, the three elements in benitoite are not the cause of its lovely blue color. We don’t know what color benitoite would be if a trace of iron was not part of its atomic structure.

Though iron is necessary for benitoite to have color, iron does not act alone. Iron’s valence electrons combine with titanium, and the silicate’s oxygen, in doing what we call charge transfer, which gives benitoite its blue color. Iron takes in some light energy as it enters the gem. Its two valence electrons use that energy to shift to oxygen, which then shifts electrons to titanium, all of which take light energy, which affects the light emerging from the gem with good blue color. All three of the elements, including iron, are important for this gem to have its blue color.

Iron, as a chromophore, often acts alone to cause color. We are all familiar with green jade, we call nephrite. Nephrite is not a single mineral. It is a tangled mixture of two minerals, actinolite, and tremolite. There’s another mineral, jadeite, and it is a single mineral.