While he was not strictly a gemmologist, it makes sense to begin this series by introducing man considered to be the founding father of crystallography - René-Just Haüy.
Haüy was a French priest and mineralogist, born in 1743. The son of a poor linen worker, it was only after the prior of the abbey at his church noticed Haüy’s intellect and keen interest in learning that an education was arranged for him.
He served as a professor at the Collège de Navarre, before becoming a professor of mineralogy at the Museum of Natural History in Paris in 1802 and later at the Sorbonne in 1809.
In one version of the tale, a critical moment behind the discovery of crystalline structure occurred when Haüy accidentally dropped a calcite crystal onto the floor – shattering it on impact. If you’ve seen for yourself the step-like structure of a broken piece of calcite, you can see why this specimen piqued Haüy’s interest in the internal arrangement of crystals.
Upon examining the fragments of calcite, he noted it “had a single fracture along one of the edges of the base. I tried to divide it in other directions and I succeeded, after several attempts, in extracting its rhomboid nucleus”. Haüy had discovered ‘cleavage’.
By continuing to break calcite into smaller pieces and observing the same step-like feature, he concluded it had the same internal structure regardless of its size.
Prior to Haüy's research, the understanding of crystal structure was limited to external morphology. An example of this is the important contribution Nicolas Steno made to the world of mineralogy with his observation that for a particular mineral, the angles between corresponding faces of a crystal are constant regardless of size, origin, or habit.
Following his observations of how crystals cleaved, Haüy devised that all crystals were periodic, with a repetitive pattern formed by the stacking of what he termed molécules intégrantes or integral molecules.
These ‘integral molecules’ are known to be unit cells. A unit cell is, by today’s definition, ‘the smallest unit of a structure that can be infinitely repeated to generate the whole structure’ of a crystal.
This explanation demonstrated that the external morphology of a crystal was an expression of its internal order and was enabled sense to be made of the different habits (different shapes) the same mineral may display.
For example, both an octahedral diamond and the much rarer cubic diamond have the same unit cell that has been stacked differently internally on an atomic level to achieve two different shapes externally.
This revelation contributed immensely to the separation and identification of minerals, as well as further developments in crystal symmetry.
Throughout his career, Haüy published various works contributing to crystallography and other realms of science, including the four-volume Traité de Mineralogie (1801).
Accompanying this Traité was a fifth volume - an atlas - complete with almost 600 diagrams and illustrations of the many crystal structures he had identified.
Believed to be the same structures depicted in this volume, Haüy also had 597 crystallographic shapes commissioned as wooden models.
This was done to showcase the three-dimensional nature of each structure - a teaching practice many student gemmologists still encounter and appreciate today.
Reading of his contributions to the field of crystallography, some may think Haüy’s work to be a bit detached from gemmological practice today. However, as any good gemmologist would know, it is this understanding of a crystal’s internal structure that allows us to separate one gemstone from another.
Haüy’s ground-breaking study opened the door for researchers to recognise cleavage and planes of weakness, to predict in which directions a gemstone may appear different colours (pleochroism), or to understand how light may interact with a gemstone, just to name a few of the properties dependent on crystal structure.
Name: René-Just Haüy
Work: French Mineralogist
Born: 28 February 1743 at Saint-Just-en-Chaussée, France
Died: 1 June 1822 (aged 79), at Paris France
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