The term iron line refers to a narrow, diagnostic absorption band in the visible spectrum of a gemstone caused by the presence of iron ions in the crystal lattice. Iron lines are among the most useful spectral features available to the bench gemmologist, since they are produced by a small number of well-defined electronic transitions in either ferrous or ferric iron and they appear consistently in particular gem species. Reading a gem's spectrum through a hand-held prism or diffraction-grating spectroscope, the gemmologist can often identify both the species and the dominant chromophore from the iron lines alone.

Where iron lines appear

The most familiar iron lines in gemmology occur in the blue-violet region between 440 and 460 nanometres, the green region around 500 nanometres, and the red end at 580 to 620 nanometres. Different gem species show iron lines at different positions because the local environment of the iron ion in the lattice modulates the energy of the relevant electronic transitions.

In blue sapphire, three iron-related lines are typically resolvable at 450, 460, and 470 nanometres, often described as the iron triplet of corundum. Their strength varies with iron content: a Thai or Australian sapphire, with relatively high iron, shows pronounced lines, while a Kashmir or Burmese sapphire, with lower iron, may show only a weak 450-nanometre line. This pattern is one of the spectroscopic cues laboratories use in origin determination, alongside trace-element chemistry and inclusion analysis.

In peridot, divalent iron produces three broad bands in the blue and red regions, sometimes loosely called the peridot triplet, although these are broader than true iron lines and behave more as absorption bands. In garnet, particularly almandine and pyrope-almandine, three sharp iron lines at approximately 505, 527, and 576 nanometres are diagnostic of the species and are visible in almost any well-cut faceted stone of sufficient saturation.

Iron lines and origin

Because iron lines correlate with iron concentration, their relative strengths can serve as a quick proxy for trace-element chemistry. A modern gemmological laboratory will quantify iron through laser-induced techniques, but a trained eye with a hand spectroscope can often distinguish iron-rich from iron-poor material in seconds. The classic separation of basaltic and metamorphic blue sapphires by iron content follows this principle, and the corresponding strength of the iron triplet has long been used as a field test.

Iron lines versus rare-earth and chromium lines

Not every sharp absorption line in a gem's spectrum is an iron line. Chromium produces narrow lines in the red region of ruby and emerald, including the famous ruby doublet near 692 to 694 nanometres. Rare earth elements such as didymium produce sharp lines in apatite and some yellow synthetics. Iron lines are distinguished by their characteristic positions and by the manner in which they accompany broader iron-related absorption rather than the narrow doublets and multiplets of chromium and rare earth ions.

Practical use

The hand spectroscope, although superseded for laboratory work by ultraviolet-visible-near-infrared spectrometers, remains a fast and inexpensive way to confirm a working identification at the bench. A confident reading of the iron triplet in corundum, the iron lines in almandine, or the broad iron bands in peridot can rule out major synthetic and simulant possibilities and direct the next steps in identification. For the trade buyer, recognising iron lines through a desk spectroscope is a useful skill that complements refractive-index and specific-gravity measurements when sorting parcels of coloured stones.