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Gemstone Phenomena: Polymorphism

| 2 min read

The world polymorphism means ‘many forms’ and is used in several different branches of science. In gemology and mineralogy, it is used to describe the phenomena of different looking minerals sharing a chemical formula.

For example, few would confuse the gemstone Diamond with graphite, which is often used in pencils. But they both share the chemical formula C, as they are both 100% carbon. This is surprising, as Diamond is one of the hardest minerals and graphite is comparatively very soft. It’s all to do with the temperature and pressure present when the mineral is forming, and the effect this has on the crystal structure within. The atoms of a Diamond are intensely packed together due to the incredible pressure present at formation, causing them to form into a perfect cubic crystal structure. Remove some of this pressure and instead the atoms will arrange themselves into a hexagonal crystal structure which is much less tightly packed and certainly doesn’t allow light to pass through it. This gives us graphite.

Polymorphism Example

But this doesn’t just apply to simple chemical formulas like carbon. Naturally occurring aluminium silicate with the chemical formula Al2SiO5 can form as Kyanite, Sillimanite and Andalusite, depending on conditions. The Kyanite variant forms at high pressure with a triclinic crystal structure, high temperatures will cause Sillimanite to form with an orthorhombic crystal structure, and if the pressure and temperature are both low, we get Andalusite, which also has an orthorhombic crystal structure. Aragonite and Calcite also share the same chemical formula too, CaCO3. They’re both a form of calcium carbonate, but at high pressure we get Aragonite with an orthorhombic crystal structure, and Calcite is formed at low pressure with a hexagonal crystal structure. It’s a fascinating and very rare occurrence which results in some beautiful gemstones that have far more in common with each other than a first glance would suggest.

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Asterism Aventurescence Chatoyancy Fluorescence and Phosphorescence Labradorescence Tenebrescence Twinning