Geochemical fingerprinting of diamonds and other gemstones

Abstract

Gems are precious materials which are used for ornamentation or decoration. Most gems are minerals having a definite chemical composition. Examples include diamond, ruby (red corundum), sapphire (any other colour of corundum), and emerald (green chromium or vanadium-bearing beryl). Gemstones (diamond, ruby, emerald, pearl, etc.) are the seeds of many dreams and they are not only associated with the cultures but also with politics, power and economy. Gemstones are prized for their colour, lustre, transparency, durability, and high value-to-volume ratio. Some gemstones are produced from minerals that are rare in nature (e.g. brazilianite), whereas many others are produced from common minerals (e.g. quartz, feldspar, and garnet). Interestingly, a common mineral becomes a gemstone if it bears certain characteristics such as appealing colour, large size, and higher degree of transparency (e.g. amethyst – compositionally: quartz, SiO2). Importantly, it is not the mineral itself that makes it a gemstone but the characteristics of a specific sample. Therefore, a corundum sample cannot become a gemstone (ruby or sapphire) unless it forms in a favourable geological condition which allows it to acquire a suitable colour, size and transparency. Generally, the gemstone deposits are rare because the essential geological conditions required for their genesis are rarely available in nature. Such exceptional geological conditions make gem-deposits fascinating for scientific study.
Gemology, the science of gems, deals with the identification of gemstones and determining whether they are natural or synthetic. To better understand the origin of gemstones, the gemology often incorporates several fields of science. For example, crystal growth studies help to understand the growth structures and the inclusions in terms of the growth environment. These features are very useful to establish the conditions of origin of a gemstone, i.e. natural or synthetic. The geology of gem deposits is central to understanding the rare and exceptional geological conditions that give rise to gem-quality materials (e.g. uncommon major constituents, adequate chromophores, limited concentrations of undesirable elements, open space, an environment conducive to forming crystals of sufficient size and transparency), the nature of inclusions and trace elements. For example, formation of diamonds requires high pressure (~4.0 GPa) and high temperature (950-1400 °C), and local enrichment of carbon as it is a trace element in the mantle. Furthermore, most gemstones (coloured) also require that their essential elements come in contact with an appropriate chromophore (e.g. Cr in ruby or V in tanzanite and tsavorite). Recently, with the development of non-destructive trace element analysis and isotopic studies, geochemistry has become an important part of gemology. It has provided geochemical criteria that help to distinguish between natural and synthetic gems, and many a times even identify the geographical origin. An understanding of the origin of colour is very important, because the value of a gem is often related to its colour, and whether it is natural or treatment induced. Furthermore, for coloured gemstones, the presence of certain geochemical components marks a distinction between a common mineral specimen and a gemstone. For example, a grossular garnet comprising the end member components only [Ca3Al2(SiO4)3] is colourless. However, presence of vanadium (V) and chromium (Cr) ions in trace amount can turn the common grossular garnet into green gemstone variety tsavorite. Such geochemical transformation of grossular garnet to tsavorite is attributed to metamorphic fluids which mobilize vanadium and chromium from host rocks and incorporate them into the grossular garnets. Importantly, such geochemical components are responsible for colouration and thus provide a “fingerprint” for determining the provenance of the gemstone. Geochemical fingerprinting of the gemstones is also remarkable in terms of identifying the geographic origin of the gemstones. This is important because some gemstones bear significantly higher economic value if they are recognized to be coming from certain place. For example, blue sapphires from Kashmir (India) fetch higher prices than those from Burma or Sri Lanka.

Full text

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Geochemical fingerprinting of diamonds and other gemstones
Iftikhar Ahmad* and M.E.A. Mondal
Department of Geology, Aligarh Muslim University, Aligarh-202 002
*Email: ifahmad@msn.com
Gems are precious materials which are used for ornamentation or decoration. Most gems are
minerals having a definite chemical composition. Examples include diamond, ruby (red
corundum), sapphire (any other colour of corundum), and emerald (green chromium or
vanadium-bearing beryl). Gemstones (diamond, ruby, emerald, pearl, etc.) are the seeds of many
dreams and they are not only associated with the cultures but also with politics, power and
economy. Gemstones are prized for their colour, lustre, transparency, durability, and high value-
to-volume ratio. Some gemstones are produced from minerals that are rare in nature (e.g.
brazilianite), whereas many others are produced from common minerals (e.g. quartz, feldspar,
and garnet). Interestingly, a common mineral becomes a gemstone if it bears certain
characteristics such as appealing colour, large size, and higher degree of transparency (e.g.
amethyst – compositionally: quartz, SiO2). Importantly, it is not the mineral itself that makes it a
gemstone but the characteristics of a specific sample. Therefore, a corundum sample cannot
become a gemstone (ruby or sapphire) unless it forms in a favourable geological condition which
allows it to acquire a suitable colour, size and transparency. Generally, the gemstone deposits
are rare because the essential geological conditions required for their genesis are rarely available
in nature. Such exceptional geological conditions make gem-deposits fascinating for scientific
study.
Gemology, the science of gems, deals with the identification of gemstones and determining
whether they are natural or synthetic. To better understand the origin of gemstones, the
gemology often incorporates several fields of science. For example, crystal growth studies help
to understand the growth structures and the inclusions in terms of the growth environment.
These features are very useful to establish the conditions of origin of a gemstone, i.e. natural or
synthetic. The geology of gem deposits is central to understanding the rare and exceptional
geological conditions that give rise to gem-quality materials (e.g. uncommon major constituents,
adequate chromophores, limited concentrations of undesirable elements, open space, an
environment conducive to forming crystals of sufficient size and transparency), the nature of
inclusions and trace elements. For example, formation of diamond requires high pressure (≈4.0
GPa) and high temperature (950-1400 °C), and local enrichment of carbon as it is a trace element
in the mantle. Furthermore, most gemstones (coloured) also require that their essential elements
come in contact with an appropriate chromophore (e.g. Cr in ruby or V in tanzanite and tsavorite).

Page | 39
Recently, with the development of non-destructive trace element analysis and isotopic studies,
geochemistry has become an important part of gemology. It has provided geochemical criteria
that help to distinguish between natural and synthetic gems, and many a times even identify the
geographical origin. An understanding of the origin of colour is very important, because the value
of a gem is often related to its colour, and whether it is natural or treatment induced.
Furthermore, for coloured gemstones, the presence of certain geochemical components marks
a distinction between a common mineral specimen and a gemstone. For example, a grossular
garnet comprising the end member components only [Ca3Al2(SiO4)3] is colourless. However,
presence of vanadium (V) and chromium (Cr) ions in trace amount can turn the common grossular
garnet into green gemstone variety tsavorite. Such geochemical transformation of grossular
garnet to tsavorite is attributed to metamorphic fluids which mobilize vanadium and chromium
from host rocks and incorporate them into the grossular garnets. Importantly, such geochemical
components are responsible for colouration and thus provide a “fingerprint” for determining the
provenance of the gemstone. Geochemical fingerprinting of the gemstones is also remarkable in
terms of identifying the geographic origin of the gemstones. This is important because some
gemstones bear significantly higher economic value if they are recognized to be coming from
certain place. For example, blue sapphires from Kashmir (India) fetch higher prices than those
from Burma or Sri Lanka.