Iron is the most prolific colour-causing element in gemstones, responsible directly or indirectly for the colour of more gem species than any other transition metal. Its prevalence is partly a matter of geochemistry: iron is the fourth most abundant element in the Earth's crust and the most abundant of the transition metals, so it readily substitutes into the lattices of silicate, oxide, and phosphate gem minerals. Its versatility as a chromophore comes from the fact that iron can occupy at least two oxidation states relevant to gems, ferrous Fe2+ and ferric Fe3+, each producing different absorption behaviour, and from the way pairs of iron ions can interact to produce additional colour effects.

Ferrous iron, Fe2+

Divalent iron, Fe2+, generates the soft greens, blue-greens, and yellow-greens of many silicates. It is the principal chromophore in peridot, where Fe2+ in the magnesium site of forsterite produces the species' characteristic yellowish green. The same ion colours olivine and the chrysolite end of the olivine series. In tourmaline it contributes to the bluish-green tones of indicolite and the greens of verdelite, alongside other transition metals.

Iron in the divalent state typically absorbs in the red and near-infrared region, transmitting the green that we see in the body colour.

Ferric iron, Fe3+

Trivalent iron, Fe3+, is responsible for many yellows. It produces the yellow of citrine, where Fe3+ replaces silicon in some interstitial sites of the quartz lattice and is converted to a colour-active form by natural or artificial irradiation and heat. It contributes to the yellows of yellow sapphire, yellow tourmaline, golden beryl (heliodor), and yellow chrysoberyl. In feldspar, dispersed Fe3+ is one of several causes of the soft yellows in some sunstone and orthoclase.

Ferric iron generally absorbs in the violet and ultraviolet region, transmitting yellow.

Iron-iron pair interactions

Where Fe2+ and Fe3+ occupy adjacent sites in a crystal lattice, an electron can hop between them under the influence of incoming light, an effect known as intervalence charge transfer. This process absorbs strongly in a particular wavelength range and can produce intense colours that neither ion would generate alone. The deep blue of iron-bearing aquamarine is produced in this way, as is the blue of certain blue tourmalines and the blue-green of some kornerupine. The well-known blue of blue sapphire, although classically attributed to titanium-iron charge transfer with both Fe2+ and Ti4+ involved, is closely related and is the textbook example of charge-transfer colouration in gemmology.

Iron in red and brown gems

Iron also contributes to many reds and browns, particularly in garnets. Almandine garnet's wine-red colour comes from Fe2+, and the various pyrope-almandine series colours grade from Fe-rich red-purple to magnesium-rich pure red. Brown to red-brown axinite, sphene, and andalusite all owe colour to iron in various site environments. The orange of fire opal is partly attributed to iron oxide nano-inclusions distributed through the silica gel matrix.

Practical implications for the trade

The dominance of iron in gem colour has practical consequences. Iron-coloured gems are often more responsive to heat treatment than chromium- or vanadium-coloured ones, since heat can shift iron between oxidation states or redistribute it among lattice sites. The classic example is heat-treated blue sapphire from Sri Lanka, where milky geuda corundum becomes a saleable blue once iron and titanium are mobilised by heat. Aquamarine's blue is intensified by heat that converts greenish iron to the more saturated blue charge-transfer state. Citrine is almost entirely a heat-treated product of iron-bearing amethyst.

Iron-coloured gems also tend to fluoresce more weakly than chromium-coloured ones, since iron quenches fluorescence. A strongly fluorescent ruby is therefore a chromium-rich, iron-poor stone, and fluorescence intensity is one of the cues separating Burmese and Mozambican rubies, where iron levels differ markedly.