Gemmological characteristics of corundum from Awissawella deposit, Ratanapura gem field, Sri Lanka

Abstract

Recently, many gem corundums have been imported from many deposits throughout the world for gem and jewelry industries in Thailand. Among these deposits, Sri Lanka deposits have been very well-known as sources of high quality ruby and blue sapphire. Geologically, Sri Lanka was a southern part of the Indian sub-continent which they appear to have separated due to shallow tectonic movement. Consequently, Sri Lanka Island contains a main basement of highly metamorphosed Precambrian rocks. These basement rocks are subdivided into three major lithological units, namely Highland Complex, Wanni Complex and Vijayan Complex. Awissawella deposit is located on the East of Colombo within the vicinity of "Ratnabura", the most famous gem deposit of the country. These deposits are closely associated with the Highland Complex. Corundum samples from Awissawella under this study can be divided, on the basis of main body color, into two varieties, i.e., yellow and light blue which both varieties contain blue color patches. Mineral inclusions are apatite, feldspar, calcite, garnet, hematite, pyroxene and diaspore. UV-VIS-NIR absorption spectra show two main different patterns depending on their varieties; yellow variety presents continuous increase of absorption toward the UV region of the spectra with a minor peak of Fe³⁺/Fe³⁺ pair at 450 nm, Fe³⁺ peak at 388 nm and band of Fe²⁺/Ti⁴⁺ pair at 565 nm caused by blue patches. Blue variety usually presents peaks of Fe²⁺/Ti⁴⁺ pair at 565 nm, Fe³⁺ peak at 388 nm and small peak of Cr³⁺ at 412 nm in bluish violet samples. Qualitative of trace element analyses show significant Fe₂O₃ contents of 0.125 to 0.940% in yellow body, 0.134-0.265% in blue body. Recalculated atomic proportions of quantitative EPMA analyses plotted in a Mg-Fe-Ti ternary diagram show that yellow components are mostly rather high Mg contents whereas blue component in both varieties have high Ti and Fe contents with low Mg contents (≤ 20%). Awissawella corundum samples could be separated based on trace element proportions, from basaltic corundums and some metamorphic corundums such as Myanmar and Songea. Regarding to heating experiments, these corundums have high potential to improve their colors. Yellow variety appear to be intensified by heating at 1,650℃ in oxidizing atmosphere; on the other hand, blue variety can be increased their blue shades by heating at 1,650℃ in reducing atmosphere.