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In order to understand how gems are formed, it is helpful to understand a little about geology. Our planet, which was created about 4.5 billion years ago, is comprised of several layers: the crust, ranging from 3 – 25 miles deep (only 1% of the Earth’s volume), the mantle (which is over 80% of its volume) and the inner part of Earth known as the core. The core has a solid inner and a liquid outer and it is this part of Earth which, for obvious reasons, is the least known and studied.Most of the mantle consists of magma, and giant segments of Earth’s crust - along with the solid upper mantle - float on this fluid liquid and move slowly over it. These big plates, some the size of continents, are known as tectonic plates. It is the movement of these plates that is responsible for many of the huge events that continually shape and reshape the landscape of Earth. These include volcanoes, earthquakes and periods of mountain building where vast ranges including the Alps, Andes and Pyrenees were formed.Earth’s crust is made up of three types of rock: igneous, sedimentary and metamorphic. Igneous rocks are those which have solidified from a molten state. Sedimentary rocks are formed due to compacted sediment layers formed by weathering, pressure and often heat. Metamorphic rocks are formed when tremendous temperature and pressure have changed the formation of either igneous or, sedimentary rock or other metamorphic rocks. While all non-organic gemstones begin their existence in one of these three rock structures, their journeys and experiences can be vastly different. There are many different scenarios and environments which lead to the creation of gemstones with differing chemical structures, crystal structures, hardness, colours and optical qualities. It is important to stress that while the creation of some gems will always be due to the same geological event, others can be created in a variety of manners. Below we detail the main monumental natural events that are responsible for the creation of some of the most valuable treasures on Earth.
Believe it or not, rain falling to the Earth’s surface millions of years ago is responsible for the creation of several vibrant gemstones that we treasure today. As water passed through cracks in Earth’s crust, it sometimes gathered different chemicals, creating a gem-forming ‘cocktail’. During this journey, the liquid dissolved different minerals and rocks; eventually it became too saturated, and came to rest in cracks and crevices. Given time and pressure, these solutions eventually turned into a new solid mineral. It is through this process that Opals, Turquoise and Malachite were formed.
As the name suggests, hydrothermal gems are created similarly to those formed from water based solutions, but with the addition of heat. Imagine the journeys of two different fluids meeting at the same point. The water based solution filters downwards through cracks and gaps in Earth’s crust, whilst hot magma solutions, often rich in fluorine and beryllium, are rising through veins and crevices. As they meet, the resulting cocktail features many different chemicals and minerals, and percolates at an incredibly high temperature. Over a period of time, as it begins to cool it produces crystals with complex chemical structures. Many Beryls were formed this way, and as you start to understand this incredibly hostile hydrothermal environment, you begin to appreciate the source of inclusions and patterns seen in Emeralds.
First of all, let’s remember that an igneous rock is one which has been formed from solidified magma. Pegmatites are created when magma is forced into crevices and other cracks and is allowed to solidify over a period of time. In very simplistic terms, gemstones formed in pegmatites are similar to those hydrothermally formed, but the main ingredient in pegmatites is magma rather than water. The magma squeezed into openings is under incredible pressure, and provides an ideal environment for crystals to grow. Some of the world’s largest pegmatites are in Minas Gerais, Brazil. These yield a magnificent array of coloured gemstones including Topaz, Aquamarine and Tourmaline. Other gems that form in pegmatites include Kunzite, Fluorite and Hiddenite.
We discussed earlier that a metamorphic rock is one that has changed its form (‘meta’ meaning changed and ‘morph’ meaning shape). Interestingly, although immense pressure and heat is needed to create a metamorphic rock, the original mineral does not actually melt. There are two different types of metamorphism and we need to have a basic understanding of both, as they are homes to different gem types:Contact Metamorphism occurs when magma (lava) forces its way into cracks and crevices of existing rock formations. Due to the incredible heat of the magma, the rocks which it comes into contact with recrystallise into new minerals. Sri Lanka (formerly Ceylon), one of the best gem treasure chests in the world, is a prime source of gems created through the act of contact metamorphism. It is incredible to think that if this event had not happened, Prince Charles would not have been able to give Princess Diana her electrifying Ceylon Blue Sapphire engagement ring! Other gems that are created through this process include Garnet, Ruby and Spinel. High in the mountains of Afghanistan, Lapis Lazuli is also created in this way. Burma sits on an area rich in metamorphic rocks and much of its high quality Ruby and Jadeite are formed by contact metamorphism.Regional Metamorphism, as its name suggests, is not confined to cracks and crevices and happens across a wider area. These rocks are often created in environments running along the edges of tectonic plates. As these plates collide with one another, the igneous rocks of each plate morph with one another under colossal pressure. As the rocks near the point of melting, and over a sustained period of millions of years, they can produce some incredibly beautiful crystal structures.As you can imagine, gems formed from the coming together of two different plates, both of which can feature many different minerals, can lead to combinations that are very specific to one area. This is, in fact, the reason why Tanzanite has only been found in one place on the planet. The morphing of two tectonic plates underneath one small area in Tanzania produced a specific cocktail that has yet to be discovered anywhere else on this planet.
So far, we have discussed events that happen in the Earth’s crust or along its inner edge. Two gemstones however, are created deep in the Earth’s mantle; Peridot and Diamond. As these are formed some 90 miles beneath the Earth’s surface, and the deepest mine in the world, the East Rand gold mine in South Africa, is only 2.1 miles deep. You might wonder how these gems are discovered? Well, they are primarily brought to the surface of the Earth or near to its surface in old volcanic pipes. These are known as kimberlite pipes.Interestingly, as Diamonds can crystallise at incredibly high temperatures in magma beneath the Earth’s crust, it is believed they may be the most common crystal on the planet, however, other than those embedded in kimberlite pipes, the remainder is just impossible to reach.
Magma flowing from volcanoes
creates igneous rocks.
Citrine is formed in geodes.
Beryl can be hydrothermally created.
Aquamarine can often be formed in a hostile
Rubies are often formed by contact
Tanzanite is formed by regional
Peridot is formed deep in Earth's mantle.
GIA qualified Gemmologists
Members of the British Jewellery Association
Members of the Coloured Gemstone Assocation