A Quantitative Description of the Causes of Color in Corundum

Abstract

The color of a gemstone is inextricably linked to its chemical composition, yet the quantitative relationship between color and chemistry is poorly understood in most cases. Here we use corundum to present a comprehensive quantitative description of the causes of color in a gem material and illustrate its predictive power. Natural corundum has six major chromophores that cause color: Cr 3+ , h • -Cr 3+ , Fe 3+ , h • -Fe 3+ , Fe 2+ -Ti 4+ , and V 3+ . We use synthetic samples doped with a single chromophore to study their light absorption behavior in isolation. Natural samples are used as well to study single chromophores, and we can subtract out the absorption of additional chromophores that might be present. Combining quantitative visible absorption spectroscopy with chemical analysis by SIMS, we are able to calculate the absorption cross section of each chromophore. The absorption cross section information is used to determine the depth of color that would occur in corundum of a given size (optical path length) containing a specific chromophore of a given concentration.

Accurate Trace Element Reporting in Corundum: Development of Secondary Ion Mass Spectrometry Relative Sensitivity Factors

“…2017), to decipher previously unexplainable colour phenomena in corundum (Emmett et al 2017b), quantify absorption cross sections for corundum chromophores (Dubinsky et al . 2020), study new intervalence charge transfer (IVCT) pairs (unpublished), check the purity of synthetically grown samples and to investigate the trace element chemistry of natural corundum from around the world (details of this work covering the SIMS quantification of all eighteen trace elements using the developed RSF values presented here will be published separately). In all cases of natural samples that were characterised, they were selected for high optical quality (lack of visible inclusions and homogeneity of colour).…”

“…The chromophore responsible for a particular visible absorption spectrum in corundum is not necessarily a simple substitution of an isovalent transition metal cation for the octahedral Al 3+ ; the chemistry of these chromophores can be quite complex and involve charge transfer pairs between species including metal cations, protons, trapped holes and vacancies where the chemistry is dictated by whether the material is donor or acceptor dominated (Dubinsky et al . 2020).…”

“…Knowledge of the trace element chemistry aids in the understanding of the absorption properties responsible for colour (Nassau 1978, Emmett et al 2017a, Dunbinsky et al . 2020) and the electrical properties of corundum (Kröger 1984a, b). For technical synthetic corundum, accurate quantification of impurities is necessary not only for controlling intentional doping, but for adequate quality control, as contamination can reduce the transparency, impart undesired colour, produce unwanted fluorescence and lead to parasitic absorption in the cases of laser gain media.…”

Raman and Photoluminescence Mapping of Gem Materials

“…Sapphires from Madagascar or Sri Lanka may show dramatic color zoning with alternating blue and colorless zones constrained to follow specific crystallographic planes and can track the crystal growth history of the gems. The blue color is derived from absorption of red light through an intervalence charge transfer between Fe 2+ and Ti 4+ sitting on adjacent octahedral sites in the corundum structure [43]; therefore, the oscillatory zoning indicates differences in the chemical environment during growth involving not only iron (Fe) and titanium (Ti) but also potentially magnesium (Mg) and silicon (Si) [44]. In Madagascar and Sri Lankan sapphires, the color zoning usually occurs with very sharp boundaries between blue and colorless zones and generally follows a hexagonal pattern when looking down the c-axis (Figure 15).…”

Intriguing minerals: corundum in the world of rubies and sapphires with special attention to Macedonian rubies

“…Despite the five d orbitals of the M 3+ ion pointing in different directions, if an isolated M ion was considered, it would not generate any colour and would remain colourless. However, the red colouration in corundum (ruby variety) involves the replacement of several hundred to a few thousand Al 3+ ppma (ppma = parts per million atoms) with Cr 3+ as impurity [28]. At these structural positions [55], the five 3d orbitals of the Cr 3+ ion, surrounded by six oxygen ions (in a distorted octahedral configuration), interact with the orbitals of the six oxygen ligands that, in turn, produce perturbations in the energy levels of the individual orbitals.…”