Why Are Some Crystals Gem Quality? Crystal Growth Considerations On the “gem Factor”

Research areas:
  • Emmanuel Fritsch
  • Benjamin Rondeau
  • Bertrand Devouard
  • Lauriane Pinsault
  • Camille Latouche
The Canadian Mineralogist
The purpose of this work is to investigate the crystal growth parameters necessary or sufficient to obtain a crystal specifically
of gem quality. We assume adequate chemistry is available. First, nucleation must occur with only a limited number of nuclei,
otherwise too many crystals will be produced, and they will be too small to be faceted into a gem. Two growth mechanisms are
readily documented for gems: Most commonly there is slow growth, driven by a spiral growth mechanism, leading to large single
individuals. There are only a few examples of fast growth leading to gem-quality edifices: examples include ‘‘gota de aceite’’
Colombian emerald or the dendritic ‘‘pseudo cube’’ for gem diamonds. We have not documented the intermediate conditions
between these two extremes in the Sunagawa diagram, which would correspond to 2D nucleation growth. The presence of
inclusions is to be limited to desirable ones. Thus, in general, a good stability of the growth interface is the best guarantee of good
clarity in the final gem. As for the interface, in general, growth conditions must be relatively stable over the period necessary to
achieve growth. Perhaps surprisingly, it has become well documented that gem-quality near-colorless diamonds may have
experienced quite a complex growth history. Therefore, the term stability has to be re-defined for each system producing a given
gem. The length of time it takes to achieve crystallization of the gem has rarely been studied or estimated. Scientific evidence
from experimental petrology and the growth of synthetic gems indicates that it does not take millions of years to grow a gem, but
that this exercise may be achieved in a week to, arguably, a few years at the most. Available free space to grow does not appear
always necessary, but it helps. Otherwise deformation, inclusions, and other negative effects may occur. Finally, no dramatic
post-growth events, such as fracturing or etching, should affect the gem crystal.