Jeweller – February 2024

REVIEW
Gems
Gemmologists who changed the game:
Donald B. Hoover
Dr. Donald B. Hoover was a distinguished figure in
the field of gemmology. He was born on June 17,
1930, and passed away on January 22, 2023, at the
age of 92.
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.
This reaction is rarely seen in natural, untreated
sapphires and can be used as an indicator of hightemperature
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.
Donald Hoover in 2007 with Grant Pearson’s opal
pineapples, mined in White Cliffs (NSW).
Hoover studied fluroescence in gemstones
such as sapphires.
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.
athryn yatt i , is a qualified
gemmologist, diamond technologist, registered
jewellery valuer, educator and member of the
Australian Antique & Art Dealers Association. For
more information on gems and gemmology, go to
www.gem.org.a
February 2024 | 29