MINERALOGY AND GEMOLOGY

Since the creation of the Earth, 4.6 billion years ago, meteorite impacts, tectonic shifts, volcanic activity, erosion and biology have modified our planet extensively. Thanks to this tremendous activity, unique crystals have formed.

Francois Farges © MNHN Collection
Meteorite “pallasite” showing peridots. Springwater, Saskatchewan, Canada. MNHN Collection, Paris.

Pressure within the earth is created by the weight of materials located above. At moderate depths, in the upper mantle, it is the realm of the diamond. Composed of carbon atoms, diamonds’ exceptional hardness and fieriness are directly related to the mantle’s pressure where they crystallise. Originally colourless, diamonds can be gifted with any color: those so-called “fancy” diamonds are usually tinted by trace amounts of other elements like nitrogen or boron.

Francois Farges © MNHN Collection
Misc fancy diamonds (31 carats). Cullinan and Kimberley, South Africa.
Gifts of R.I., Bischoffsheim, 1889 and L. Taub, 1890.
MNHN Collection, Paris.

Inside the Earth’s crust, only a few kilometers down, temperature also controls the way minerals crystallise from molten rocks. Among the many minerals able to crystallize from these magmas, quartz is one of the most common. Often associated with quartz, tourmaline occurs with a variety of amazing colours that illustrate subtle changes in its chemical composition during crystallization.

Francois Farges © MNHN Collection
Rubellite crystal (group: tourmaline, species: elbaite). Pala, California, USA. Gift of J. Pierpont-Morgan, 1905. MNHN Collection, Paris.

With pressure and temperature decreasing closer to the Earth’s surface, another key principle in “mineral-making” is transport. Elements can migrate from one location to another and, thanks to that transport, they can aggregate to make a bigger crystal.  Incredible minerals can develop in that context, such as gold and topaz. Those outcrops, called “primary”, can later be eroded and their gems transported by water into sediments, which then form “secondary deposits”. This applies to a variety of emeralds formed under moderate temperatures and pressure.

Francois Farges © MNHN Collection
Emerald crystal on pyrite and calcite (possibly reconstructed). El Chivor, Boyacá, Colombia. Gift of H.-J. Schubnel, 1987. MNHN Collection, Paris.

Transport is often facilitated by water, which is an excellent “migration agent” within the Earth’s crust. Water percolation within sediments can also form impressive veins deposited by hot and pressurized aqueous solutions creating amethyst.

Francois Farges © MNHN Collection
Amethyst gem crystals. Las Vegas, Veracruz, Mexico. Gift of the Total Foundation, 1998. MNHN Collection, Paris.

The production of oxygen by living organisms, particularly in oceans, has enriched the Earth’s atmosphere, and oxygen has deeply modified the mineralogy of the Earth’s surface by producing a huge amount of “new” minerals. Among them, two are fine gems: malachite and turquoise. Both are coloured by an oxidized form of copper that can only be found on the Earth’s surface.

Francois Farges © MNHN Collection
Malachite (cut and polished). Tourtscheninowski, Ural Mountains, Russia. Former collection of the King’s Cabinet of Natural History, MNHN Collection, Paris.

For 3.5 billion years, life has deeply modified the Earth’s surface. Not only have some organisms synthesized oxygen, but many have also produced shells and skeletons. These are formed by “bio-minerals”, meaning minerals produced by living organisms. They include ivory, coral, pearls and mother-of-pearl, among many others.

Francois Farges © MNHN Collection
Oyster shell with two fine pearls, Queensland, Thursday Island, Australia. Gift of J. Pierpont-Morgan, 1903. MNHN Collection, Paris.

With fossilization, the Earth’s materials are buried back at greater depths. Little by little, mineral accumulations are subjected to increases in pressure and temperature, inducing recrystallization: this is called metamorphism. Metamorphism can also take place when two continents collide together. This is how the rarest Mogok rubies formed, based on limestone deposited within a lost ocean.

Francois Farges © MNHN Collection
Lazurite crystals in white marble.
Sar-e-Sang, Badakhshan, Afghanistan. MNHN Collection, Paris.