EXTENDED ABSTRACT
This paper will discuss techniques developed while researching practical aspects for determining the apparent and true dispersion of gemstones using a gemmological refractometer. Each gemstone has a distinct dispersion, since dispersion is a property related to a variation in refractive index with a change in wavelength.

Low-dispersion barium glass yields a colourless refractometer glass that allows measurement of refractive index in deep blue light below 500 run when used in association with the hemisphere or hemicylinder prism refractometer to which the wide field eyepiece of the Eickhorst SR/M refractometer has been attached. The Eichorst refractometer, with a visually expanded scale, is the preferred instrument for measuring dispersion because apparent refractive index can be measured at wavelengths other than that of the standard sodium emission light source at 589.3 nm. Accuracy, not available on sixty-degree exit prism refractometers, is available for refractive index determinations at many wavelengths not previously available on refractometers.

The choice of lighting techniques can improve the accuracy of refractive index determinations when used in association with appropriate sources of narrow bandwidth blue 486 run and red 656 nm wavelengths. These measuring techniques, when combined with extrapolation of a Rl-wavelength dispersion curve using Sellmeier's linear conversion, permits the extrapolation of refractive indices at B (686.7 nm) and G (430.7 nm) wavelengths and allows comparisons of these data with values previously published in the gemmological literature

This research suggests that existing B to G lists for the dispersions of gemstones leave much to be desired with respect to their accuracy. Dispersion at II-G interval was rarely measured. It was extrapolated by various techniques that provide conflicting data. The B-G interval for the measurement of dispersion that was adopted by gemmology produced a larger value, but this is of no practical use to gemmologists.

A new list of values for the dispersions of gemstones, measured at the CT (656.3 - 486.1 nm) interval with the assistance of the gem refractometer, is presented in table 3 of this paper.

Measurements of dispersion, with the gemmologist's refractometer, produces apparent dispersion. However, a means of converting this to true dispersion, after extrapolation, can be achieved by Suhner's technique of subtracting apparent dispersion from the sum of true plus apparent dispersions.

GEMSTONE RESOURCES OF CHINA

ABSTRACT
The Peoples' Republic of China has a large and ancient land mass that is host to vast, largely untapped reserves of gemstones. In this paper the author will briefly describe and illustrate the occurrences of gemstones that are relatively common in China, before providing useful tabulation of the occurrence of these and other gemstones in this historically ancient country.

GLASS-FILLED RUBIES
- Clarity-enhanced rubies with glass-forming additives

ABSTRACT
This paper described the filling of fractured rubies with lead, bismuth, tantalum and other oxides-additives including chromophores. In December 2004 a series of experiments were performed at the author's gem treatment lab in Bangkok, using various methodologies practiced commercially in Thailand. The experiments showed that certain combination of metal oxides can effectively fill surface reaching cavities in rubies at relatively low to mid range temperatures of 900-1300 ^oC. Starting material consisted of rubies with surface-reaching cracks. After treatment, the cracks were filled with the clarity and overall appearance of these rubies being considerably improved. The structure of most of these fillers is amorphous, thus these rubies may be referred to as glass-filled rubies. In Thailand these treated rubies are known as ruby-star or by their alternative name pao-mai, meaning new burn in the Thai language. This treatment is easily identified with a gemmological microscope; where characteristic inclusions such as the 'flash-effect', irregular-shaped reflective platelets/voids, 'frosted' areas, gas 'bubbles' and other features are observed. Stability and durability of the glass-filled rubies may be somewhat affected due to recutting and polishing and jewellery repairs. Disclosure of this treatment is mandatory, and their selling price in the market is relatively low.

¹Alessandro Pavese, ²Loredana Prosperi, ³Monica Dapiaggi

¹Professor of Mineralogy, Università degli Studi di Milano, Milano, Italy
²Gemmological Laboratory, Instituto Gemmologico Italiano, Sesto San Giovanni, Italy
³Senior Researcher, Dipartimento di Scienze della Terra, Università degli Studi di Milano, Milano, Italy