Hoover completed a Doctor of Science equivalent to a PhD at the Colorado School of Mines in Denver, US. Hoover worked for a long time with the United States Geological Survey as a senior scientist specialising in seismology, volcanology, geothermal energy, and nuclear waste disposal.
After he retired from USGS in the 1980s, he delved further into gemmology. He was known for his energy and enthusiasm, particularly regarding geological and gemmological discovery and research. Hoover was also a member of the editorial review board of The Australian Gemmologist.
One of Hoover's notable contributions is his work on the thermal properties of gemstones. Thermal inertia is a measure of how quickly the surface temperature of a material can be changed with the application of heat.
It depends on the material's thermal conductivity, specific heat, and density. A material with high thermal inertia will resist changes in temperature and feel cold to the touch. In contrast, a material with low thermal inertia will easily change temperature and feel warm to the touch.
Hoover compiled a comprehensive table that arranged materials based on their thermal inertia. This work has been instrumental in gemstone analysis, providing a valuable parameter for accurate, quantitative probes.
Fluorescence
Hoover also made significant strides in the study of fluorescence in gemstones. Fluorescence is the emission of visible light by a gem material when exposed to radiation of shorter wavelengths, such as ultraviolet (UV) light.
Some gemstones, such as synthetic and heat-treated sapphires, show a chalky blue to green fluorescence in short-wave UV light caused by isolated Ti 4+ ions or Ti–Al vacancy pairs in the crystal structure.
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| L to R: Hoover studied fluroescence in gemstones such as sapphires. |
This reaction is rarely seen in natural, untreated sapphires and can be used as an indicator of high-temperature heat treatment.
Hoover’s research, published in a paper entitled ‘Fluorescence excitation-emission spectra of chromium-containing gems,’ explained the effectiveness of the crossed filter method in examining emission spectra in gemstones such as rubies.
This study has been pivotal in understanding how fluorescence can be used as a gemmological tool.
The crossed-polar method uses a circular polariser filter in front of the UV lamp and another in front of the camera lens, aligned perpendicular to each other.
This eliminates the red fluorescence of ruby and enhances the contrast of the chalky blue fluorescence, making it easier to see the zoned patterns that reflect the original crystal growth structure.
This method can also be used with a Schott BG-12 filter, which transmits the blue fluorescence and blocks the red fluorescence.
Gemstone testing
Hoover has also been involved in preliminary investigations of gemstone testing using strong rare-earth N52-grade neodymium magnets.
These magnets detected 35 per cent more HPHT synthetics than traditional magnets. His innovative research has laid the groundwork for further studies on using magnetism in gemstone identification.
In another study, Hoover derived garnet composition from magnetic susceptibility and other measurable properties. The garnet compositions derived from this method corresponded closely with results obtained from chemical data, demonstrating the accuracy and reliability of this approach.
His research covered many areas, including opal from all over the world and his thirst to discover the origin of Brazilian diamonds.
Hoover's contributions have had a profound impact on the field of gemmology. His research has not only advanced our understanding of gemstones but also provided practical tools and methodologies for gem identification.
His work continues to be referenced in contemporary gemmological studies, underscoring its enduring relevance. His legacy inspires current and future gemmologists, ensuring his impact will be felt for years.
Name: Donald B. Hoover
Work: Gemmologist
Born: 17 June 1930
Died: 22 January 2023 (Age 92)
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