Basic Gem Testing

Posted on September 1, 2021 by Cara Williams, FGA

Easy Guide to Separating Blue Topaz from Aquamarine using the Chelsea Color Filter One-step testing is a tempting proposition; however, it requires extensive knowledge and experience to perform the minimum number of tests required for accurate and conclusive identification. Once-diagnostic tests; such as looking for the 653nm absorption in zircon, horse-tail inclusions in demantoid garnet, or a bull’s-eye interference figure in quartz; are not as fully diagnostic as once presumed. Today’s marketplace offers an enormously broad variety of gem materials, many of which were rarely ever seen faceted or in jewelry. While advanced instruments designed for gemological purposes become more available and affordable, the coveted “black box” that offers simple, accurate and conclusive identification remains a long way off. This series aims to bring attention to the potential pitfalls of various quick tests and how to avoid them. The Chelsea color filter (CCF) was developed by Gem-A instructors Basil Anderson and James Payne in 1934 to separate natural emerald from the common imitators of the day. Not long after, synthetic emeralds entered the market and were found to offer similar results. The Chelsea filter (Figure 1) remains a valued pocket instrument because of its ease of use with both rough and set gems, and because there are numerous other applications. One of the best applications is in separating aquamarine and blue topaz. As most GemGuide readers already know, the iron in the aquamarine will make it appear greenish under the CCF, while the irradiated shades of blue topaz will not appear greenish, but varying shades of colorless to pale peach. Figure 2. The number of different variations in radiation treatments and shades of blue topaz make it less consistent in the precise reaction, but those that appear most like aquamarine will appear colorless to a fleshy, pinkish color. Another common aquamarine imitator, light blue synthetic spinel, is colored by cobalt and so turns a strong red to pink under the CCF. FIGURE 1. The dynamic duo—Chelsea color filter and London dichro-scope. In some cases, one pocket instrument can reinforce the other. Photo by Cara Williams. Vivid blue glass that imitates sapphire is also colored by cobalt and yields a strong red reaction. However, there is also pale blue or light bluish-green glass – sometimes looking like the glass of an old Coca Cola bottle and colored by iron – which may give a CCF reaction similar to aquamarine. Figure 3. While encountered less often, these are found in many vintage rings and will occasionally come...

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