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Amethyst
Amethyst is the violet to purple variety of crystalline quartz5.
Formation:1
Amethyst can form in igneous, Metamorphic, and sedimentary rock, the largest production of
amethyst found in the southern state of Brazil, Rio Grande do Sul and Uruguay and is formed in
geodes in the igneous rock basalt. Amethyst geodes are hollow rocks with Amethyst crystals lining the inner walls. Amethyst geodes form in a two-step process firstly there is the formation of the cavity and then the formation of the crystals.
The first step in the natural process that creates Amethyst geodes is the formation of gas cavities in magma (lava flowing under the ground). The gas cavities can form from bubbles (just as carbonation causes bubbles in your Coca-Cola). Some scientists theorize that the cavities form when cooling magma flows near tree roots or other things sticking in the ground.
The cooling lava hardens completely before filling in around the outcrop, creating a cavity.
Once the cavities have hardened the remaining silica-rich liquid that the gas bubble was made up of begins to crystallise. Over time, this liquid forms six-sided pyramids (rhombohedrons) crystals of Quartz. Crystals with a colour that range from light lilac to deep purple are formed when there is a trace of Iron in the liquid, resulting in Amethyst geodes.
Colouration:2; 3; 4
The colouration of amethyst comes from colour centres in the quartz crystal structure and irradiation. Colour centres are defects in the crystal structure that absorb light and dictate what colour the eye sees. There are various kinds of colour centres such as vacancy colour centres, electron colour centres, vibronic colour centres and hole colour centres. Hole colour centres are more common and are the kind of colour centre that affects Amethyst.
Hole colour centres are caused when the crystal structure is exposed to irradiation, artificial or natural such as from nuclear decay in the surrounding rocks, and the irradiation causes an electron to be displaced from its original orbit creating a vacancy or ‘hole’ within the crystal structure. In Amethyst the colouration is due to trace amounts of Fe+3 (Iron) being present in the quartz crystal lattice and replacing some silicon atoms combined with irradiation causing the Fe+3 to lose an electron becoming Fe+4. This +4 iron absorbs certain wavelengths (357 and 545 nanometres) of light causing the Amethyst colour. The quartz needs to contain the right amounts of iron and have been subjected to enough natural radiation to cause the colour centres to form.
Mining:5
Amethyst mining in Rio Grande do Sul (currently the largest production area of Amethyst in the world) is carried out by underground tunnel mining into the hard basalt formed due to large horizontal lava flows that follow the topography caused by extensive volcanic activity.
Due to Amethyst forming in this hard-igneous rock (basalt) heavy duty equipment and small amounts of explosives are needed to make the tunnel and remove the dense rock which follows the flow path. Once an Amethyst geode is found, a small hole is punctured in the surface and a light is lowered into it so the quality can be accessed, hand tools are then used to chisel the Amethyst geode out of the
surrounding basalt intact, if possible. Because the basalt is so hard, removal of a large, intact
Amethyst geode may take as long as a week.
1 https://sciencing.com/how-amethyst-geodes-formed-4913351.html
2 http://www.geologyin.com/2018/05/what-causes-purple-color-of-amethyst.html
3 Gem-A Diploma in Gemmology 2009 pg. D7-26
4 http://www.quartzpage.de/amethyst.html (Lehmann and Moore, 1966)
5 Amethyst Mining in Brazil https://www.gia.edu/doc/Amethyst-Mining-in-Brazil.pdf (David Stanley Epstein, 1988)