Abstract
The monetary value of gemstones is based on five variables: rarity, cut, weight, color and clarity. The latter refers to internal impurities and defects. Fashion may also dictate demand and price. To enhance some of these features and value, gemstones are treated. Disclosure or nondisclosure thereof has been controversial and affected consumer confidence. Most of these treatments are difficult to detect with the naked eye and accurately quantify with traditional optical and analytical methods. X-ray micro computed tomography (micro-CT or μCT) is proposed as a relatively low cost, physically non-destructive and complementary method to detect and quantify clarity enhancement and also to provide a unique 3D fingerprint of each gemstone. A collection of 14 cut colored gemstones was selected. Micro-CT scans allowed fracture detection, their distribution and calculation of filler volume as well as 3D mapping of inclusions, surface and internal imperfections and artificially induced modifications. As a result the method allows the construction of a digital twin. X-ray exposure could however induce unwanted color changes. This effect was minimized or eliminated by optimizing dosage and exposure time.
Highlights
The fracture filling material is clearly visible by its higher density compared to that of the gemstone host
This study demonstrates that X-ray micro computed tomography can make a positive contribution to the characterization and classification of colored gemstones
A CT-scan and supporting software allows the detection of fracture filling material and their volumetric determination, provided the density difference between the host stone and the filling material is adequate such as for example lead glass filler
Summary
The clarity of a gemstone decreases with the presence of small cracks or fractures and mineral inclusions. Inclusions may decrease in size or diffuse into the host material, thereby improving its clarity. Fractures are commonly filled with a lead-based glass of a high refractive index similar to that of corundum and can alter highly fractured, almost opaque, red corundum with no gem value into translucent and even transparent gem rubies. The filler may vary from solids to liquids and are not always stable or durable. This results in damage to gems and loss of value during jewelry manufacturing or normal use during its lifetime [6,7,8]
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