Mong Hsu Blue Colour Core — A Diagnostic Zoning of Burmese Ruby
Mong Hsu Blue Colour Core — A Diagnostic Zoning of Burmese Ruby
The blue-to-greyish-blue central zone of unheated Mong Hsu material, target of high-temperature borax treatment
The Mong Hsu blue colour core is a distinctive blue to greyish-blue zone occupying the central portion of unheated rubies from the Mong Hsu deposit in Shan State, eastern Myanmar, surrounded by purplish-red zones in the outer regions of the crystal. The blue core is the most diagnostic feature of unheated Mong Hsu material and the principal reason that the deposit's ruby is routinely subjected to high-temperature heat treatment with borax flux: the treatment dissolves or homogenises the blue core, producing a more uniformly red appearance suitable for the international ruby market.
Origin of the blue core
The blue core results from differential trace-element distribution during the crystal growth of Mong Hsu ruby. The central zone is enriched in iron and titanium relative to chromium, with the iron-titanium pair producing blue colour in corundum through intervalence charge transfer (the same mechanism responsible for blue sapphire). The surrounding zones are enriched in chromium and depleted in iron and titanium, producing the characteristic red colour of ruby.
The zoning pattern reflects the chemistry of the growth environment, which evolved through the formation of the host marble and the deposition of the corundum crystals from circulating fluids. The early-formed central zone captured higher iron and titanium concentrations from the growing fluid; the later-formed outer zones grew from fluid depleted in those elements and enriched in chromium. The result is a striking two-tone presentation that, while gemmologically interesting, is commercially undesirable in a ruby intended for the international jewellery market.
Treatment to remove the blue core
The standard treatment for Mong Hsu ruby is heating at temperatures above 1,500 degrees Celsius — often in the 1,700 to 1,800 degree range — in the presence of borax (sodium tetraborate) flux. The high temperature dissolves the iron-titanium clusters responsible for the blue colour, redistributing the trace elements within the crystal lattice and producing a more uniformly red colour. The borax flux serves a secondary function: it melts at the treatment temperature, flows into surface-reaching fractures, and cools to fill the fractures with glassy material that improves apparent clarity.
The treatment is highly effective at removing the visible blue core in most material. In some heated stones, however, relict blue cores remain visible under magnification or in transmitted light, particularly when the treatment was incomplete or the original blue zone was particularly deep. The presence of a relict blue core in a heat-treated ruby is a diagnostic feature confirming Mong Hsu origin even in the absence of other identifying features.
Diagnostic significance
The blue core is one of the most reliable identifying features for Mong Hsu provenance, both in unheated material (where it is overtly visible) and in heated material (where relict cores or characteristic post-treatment chemical signatures persist). Laboratories examining a ruby for origin attribution will look for the blue core, the post-treatment relict features, and the characteristic flux-filled fractures and residues that accompany the standard Mong Hsu treatment regimen.
For the bench gemmologist, the blue core in unheated Mong Hsu material is unambiguous when present and distinguishes the source clearly from Mogok ruby (which does not show the same zoning pattern), Thai-Cambodian basaltic ruby (different inclusion suite and chemistry), and Mozambique ruby (different zoning and trace-element profile). See also: Mong Hsu ruby; Mong Hsu flux remnant; flux heating.