TRACE ELEMENT AND CHROMOPHORE STUDY IN CORUNDUM -- APPLICATION OF LA-ICP-MS AND UV-VISIBLE SPECTROSCOPY

Color in a gemstone captures our first attention and is critical to the gem’s appeal and value. Colors in some of the most desired gemstones such as ruby, sapphire, emeralds, tourmaline and jadeite owe their allure to only minute amount of foreign elements (chromophores), often at a level of less than a thousandth of the major elements that these materials are made from.

Trace element analysis by LA-ICP-MS has been refined to cover virtually all elements and with sensitivity at the level of parts-per-million and spatial resolution routinely in the range of 10-100 microns. Furthermore, custom instrumentation has been developed for chromophore analysis by UV-visible spectroscopy in specific area or zones of materials where chemical make up is precisely known from LA-ICP-MS analysis. Chromophores properties are properly established from the complementary information of LA-ICP-MS and UV-visible spectroscopy.

Chromophores in corundum (ruby and sapphires with various colors and hues) have been systematically studied. Primary chromophores, which are responsible for all corundum colors, i.e., Cr³⁺ for red, Ti⁴⁺-Fe²⁺ charge-transfer pair for blue, Fe³⁺ for yellow, and trapped-holes introducing an orange hue, exhibit dramatic difference in their effectiveness producing colors. For instance, a trapped-hole induced color is nearly three orders of magnitude more effective in producing color than an isovalent ion Fe³⁺ (sane electric charge as the native Al³⁺) in corundum. Alternatively, for the same amount of chromophores of trapped-hole and Fe³⁺, the trapped-hole color would be a thousand times more saturated that that of Fe³⁺ color.

Similar to color palettes in painting, chromophores are additive in producing various saturations or secondary colors in corundum. Once primary chromophores are thoroughly studied, the full rainbow colors available in corundum are mapped for their chemical makeup and UV-visible spectra. This becomes the foundation to understanding observed colors and to detecting possible treatment. The developed technique and methodology are fully applicable to study of color origin of variety of gemstones.