부산대학교 도서관

In recent years, a purple opal with special patterns has appeared in gemstone markets. The purple opal is composed of purple fluorite mixed with white and transparent silica minerals. The silica minerals consist dominantly of opal with minor traces of chalcedony and quartz. Studies have shown that the formation of these gemstones occur through the gradual replacement of fluorite with silica minerals, hence giving them the alternative name of opalized fluorite. There are two famous localities for this gemstone: (1) Spor Mountain, Utah, USA and (2) Jalisco, Mexico. Gemstones originating from the former locality are commercially known as 'Tiffany Stone', while those found in the latter are known as 'Morado Opal'. The appearance and mineral composition of these two gemstones are very similar, so it is confusing in the gem market. In this study, we used Geiger counters and long-short-wave ultraviolet lamps, popular in identification institutions, along with a polarized microscope, Micro-XRF, and Micro-Raman to distinguish the differences between them. The XRF results reveal mostly matching elements in the gemstone compositions of the two localities. However, the Tiffany Stone of Utah contains arsenic, and its uranium content and radioactivity are overall higher than the Morado Opal. Fluorescence is evident in samples from both localities. The Tiffany Stone shows blue fluorescence under longwave UV light, with a small portion of samples presenting green fluorescence under shortwave UV light. On the contrary, the Morado Opal shows very strong green fluorescence under shortwave UV light. As for the main difference in the crystal structure, a puzzle-like, brecciated feature is found in the Tiffany Stone, with its fissures filled with euhedral purple fluorites. On the other hand, euhedral fluorites were seldom observed in the Morado Opal, with only the occasional observation of platy pseudomorphous purple fluorite.