Summary

Large quantities of rubies from the new deposit at Mong Hsu have been widely available since 1992. Most are heat-treated before they enter the jewelry trade. Mong Hsu rubies are easily recognized by their distinctive microscopic properties. They have a number of features that thus far have not been reported for rubies from other localities. These include:

a) distinct color zoning confined to specific growth structures, with one or two violet "cores" surrounded by a red "rim"; b) spectroscopic features in the red to yellow portion of the visible spectrum, with absorption bands that are removed during heat treatment to change the cores from violet to red; c) presence of whitish particles in certain growth zones, formed as a result of heat treatment.

Color zoning in Mong Hsu rubies is closely related to a complex chemical zoning confined to growth layers formed parallel to the basal pinacoid, to the positive rhombohedron, and to two hexagonal dipyramids. There is a distinct growth sequence whereby red and violet areas formed in various growth cycles, with a specific habit change between different growth zones.

The formation conditions of rubies have been reconstructed following the study of mineral associations and fluid inclusion analyses. Mong Hsu rubies were formed at 2-2.5 kbars and at temperatures between 500oC and 550oC. The fluids were found to be water-bearing and multi-volatile CO2-rich. Fluorite inclusions in the rubies indicate that fluids must have contained some HF. For further details see the role of fluorine in the formation of color zoning.

The distinctive properties of Mong Hsu rubies are useful in separating faceted samples from their counterparts in also in establishing the locality of origin. The most prominent diagnostic properties of faceted, heat-treated Mong Hsu rubies require careful microscopic examination, using immersion techniques in conjunction with fiber-optic illumination.

Key features include growth structures confined to a distinct color zoning between cores and rims; different types of whitish particles and whitish streamers are also of diagnostic value. Specialized laboratory techniques, such as XRF analysis and IR spectroscopy, provide additional diagnostic information.

Problems for the trade arise, however, from the large numbers of stones with fractures that appear to have been filled with a foreign material, especially partially healed fractures with glassy and/or crystalline fillers.