Characterization of dark core and blue patch in Mong Hsu ruby

Mong Hsu rubies from Myanmar include typically the dark core and blue patch but most of the previous research has merely focused on the dark core. This work is aimed to understanding clearly the characteristic of the dark core and blue patch of Mong Hsu rubies. It was found from the FTIR analysis that the dark core and blue patch region showed the absorption peaks of boehmite ($1986\;cm^{-1}$), diaspore ($2115\;cm^{-1}$) and water ($3400{\sim}3900\;cm^{-1}$) but the absorption peaks of O-H stretching ($3309\;cm^{-1}$ and $3078\;cm^{-1}$) were found only in blue patch region. The UV-VIS-NIR analysis of the dark core region revealed the stronger absorption of $Cr^{3+}$ at 405 and 554 nm compared to the blue patch region. In range of 600~800 nm, the UV absorption characteristic at 659, 675 and 693 nm assigned to $Cr^{3+}$ of core group samples is distinct from that of blue patch. The SEM-EDS examinations disclosed the existence of lots of micro-cracks and pores in the core regions compared to blue patch region.