Zambian Emerald formation
Zambian Emeralds have extraordinary diaphaneity. Whereas some Emeralds might be cloudy, Zambian Emeralds are known for giving an uninterrupted view of the gem’s one-of-a-kind inclusions. This is a unique feature for Emeralds from this region, but why?
To understand what makes Zambian Emeralds so unique, we must understand how Emeralds usually form. Emeralds are a variety of the mineral Beryl, which is a beryllium aluminum cyclosilicate ((Be3Al2(SiO3)6). In its purest form it would produce the gem Goshenite. Emerald needs impurities in order to be green. These are usually either vanadium or chromium, but in Zambian Emerald’s case, there is the addition of iron. This gives Emeralds from this area an unusually rich color and gives them their famed bluish green hue.
The chemical ingredients that form Emeralds are rare. Beryllium occurs only rarely in the earth’s crust (around 2 to 6 parts per million), and usually not in the same places as chromium and vanadium. This is what makes Emerald so rare.
Temperature versus pressure
Not only does there have to be just the right cocktail of elements, there also has to be just the right temperature and pressure to form. Emeralds need room to grow into and this depends on the pressure in the area. If the pressure is too high, then there won’t be space in the surrounding rock and the Emerald will form with more cracks and inclusions within it. The lower the pressure, the better the diaphaneity and clarity, and vice versa.
There is also a rule that runs through the world of chemistry that the higher the temperature, then the lower the pressure needs to be for chemical reactions to occur. Emerald formation is the perfect example of this - if the temperature is higher, the pressure can be lower for the gemstone to form.
Given that we know that the finest Emeralds form in the lowest pressures, the ideal conditions for formation are those of the lowest possible pressure at a higher than usual temperature.
Zambian Emeralds formed during an extraordinary geological event that allowed them to form at high temperatures and low pressures, giving them their electrifying diaphaneity.
As with the genesis of most gemstones, Zambian Emerald begins its story at the time of the supercontinent Gondwana. Usually, if you imagine tectonic plate movement, you might think of the huge forces involved in the Indian plate colliding with Eurasia and creating the Himalayas, or oceanic plates separating to form the Mid-Atlantic Ridge. These enormous forces were hugely powerful, but neither would have caused exactly the right conditions to create Zambian Emeralds.
In order to create Zambian Emeralds, there had to be just the right combination of trace elements to form the gem and just the right temperature to start crystallization off and just the right pressure to allow them to grow.
So how did these prefect circumstances come together in Zambia?
Movement of cratons
The answer lies in how the cratons in Gondwana moved 550 million years ago. Cratons are the oldest parts of the earth’s crust. They are usually found in the middle of tectonic plates, having survived several cycles of rifts and merges of the surrounding continents. Some of the oldest of all are found in Africa.
550 million years ago, it is thought that these cratons began spinning, like the cogs of a watch, in opposite directions. The Congo Craton rotated alongside the neighboring and static Kalahari Craton. Added to this, the Indian and Antarctic Cratons moved closer and began rotating against one another.
The friction generated between these four cratons would have been enormous and would have created areas of varying pressure. As rocks moving into these ‘pinch points’ between cogs were compressed, there would have been enormous pressure and heat.
Conversely, at the other side of these pinch points, the pressure would be released, as the landmass had more room and the rocks begin to cool. These ongoing changes meant that rocks were forming from magma and cooling at different rates across small areas.
As rocks solidify from magma, water is released, which fills any cavities in neighboring rocks. This meltwater contains huge numbers of rare dissolved minerals – the building blocks for gemstone formation. This is why some areas of Africa are so densely populated with gemstones. These cavities are known as pegmatites and are well known for creating the ideal growing conditions for gemstones to form. This movement of cogs created areas of low pressure and is an extremely rare (perhaps even unique) geological occurrence. This means that Zambian Emerald formation was incredibly unlikely. In these areas of densely concentrated minerals, in areas of low pressure, in the heat caused by this rotation of cratons, Emeralds began to form. But in order for them to keep growing, something even more extraordinary needed to happen.
As these cogs were turning, the East and West halves of Gondwana collided.
This created a massive super-mountain in an incredibly short timeframe, which would have dwarfed any of the mountains we are familiar with on the planet today. Since vegetation hadn’t developed on the planet at this time, trees and grasses that would have protected the mountain from erosion were absent. This meant that it was eroded away by weathering in an extremely short span, producing sediment of unusually low density. This sediment gradually became buried under layer-upon-layer of new sediment. These low-density rocks allowed the Emeralds room to grow. Whereas in Colombia, for example, the Emeralds formed in pockets within very dense rocks, thus limiting their size, in Zambia, this soft sedimentary rock could give way as the crystals grew larger in this high temperature, low pressure environment. These low pressures allowed Zambian Emeralds to grow with the extraordinary diaphaneity they are famed for.
Through these unique conditions, Mother Nature created the perfect cocktail of elements and environment for Zambian Emerald to grow with better than usual diaphaneity and larger than usual crystals.
With their unique formation giving them extraordinary beauty, Zambian Emeralds were uniquely placed to take on the gemstone industry.
Read part 4 here or shop Zambian Emerald jewelry here.