July 08, 2024

How are authentic gemstones formed or created?

For as long as humans have been finding and admiring gemstones, we have been captivated by their allure and symbolism. In fact, the oldest piece of gemstone jewellery ever found was a bracelet dated to an astonishing 40,000 years ago. It was found in Siberia, and although it is made of Chlorite, not a precious stone today, it was polished and worn by revered members of the society of that time, indicating that gemstones have always been regarded as a valuable commodity.

Since then, gemstones have adorned everything from royal crowns to custom engagement rings, but only since the advent of sciences like geology have we learned the fascinating story of how gemstones are formed. Understanding their origins and the processes behind their development has only deepened our appreciation of these natural wonders. Not only that but also how it has informed our choices when choosing them for an item of fine jewellery or bespoke engagement rings

Let’s then dig into the formation processes of authentic gemstones, the environmental factors that influence their creation, and answer how gemstones are formed.

What are authentic gemstones?

Authentic gemstones are rare, naturally occurring mineral crystals that form over millions of years under very specific geological conditions. ‘Authentic’ implies it was created naturally, and not a synthetic material - after all, it is this knowledge, that it is natural, that makes us value it. You may have heard the terms ‘precious’ or ‘semi-precious’ when describing gemstones, and while they are all considered special and hold intrinsic value. semi-precious refers to more common gemstones, such as varieties of Quartz. Examples of precious gemstones include: Diamonds, Emeralds, Sapphires and Rubies.

The environments where gemstones form

The Earth is made up of many, many different layers of rock and geological material. It’s in the Earth’s rocky outer layer, the crust, in which most gemstones are formed, about 5 to 70 kilometres underground. In order for certain types of minerals to form into crystals, a process called crystallisation, involving five crucial processes, must combine and align; space, pressure, temperature, mineral ingredients and time. It is this combination of minerals and environmental conditions that creates different gemstone varieties. The question is: how do each of these five elements contribute to create what is eventually the centre-piece in that pendant you love so much?

Space

This may seem obvious, sufficient space is needed for a gemstone to form. During the formation process, minerals grow, so the space into which they form determines how they form - both in terms of their eventual size and their shape. The Earth’s crust is full of cavities and fractures within rocks, and these are the spaces into which these minerals expand. 

Pressure and temperature

We’ve all heard the saying that diamonds are formed under pressure, and that is indeed true. Diamonds are created within the earth's mantle with up to 80kb of pressure. The Diamonds are then brought up to the earth's surface in volcanic eruptions. Interestingly, the Diamonds are older than the rock they are found in. The rock they are found in is called Kimberlite or Lamprolite. On the other hand, lower pressures and lower temperatures closer to the Earth’s surface can create Opals.

Mineral ingredients

As well as space, there has to be the right combination and quantity of chemical elements and mineral ingredients available for gemstones to form. For example, Emeralds are formed as Beryllium Aluminum silicates with the presence of Chromium to make it green. On the other hand, if Beryllium aluminium silicates with iron, it could make Goshenite or Aquamarine. Diamonds are formed from carbon, Ruby from a combination of Aluminum Oxide and Chromium, and Sapphires are created when Aluminium Oxide combines with Ti + Fe. Without these mineral ingredients, natural, authentic gemstones would not be able to form.

Time

As with the right marriage proposal, timing is everything. The formation of gemstones happens over a massive time-scale, usually taking between 200 million  and 3.5 billion years to form. The more time, the larger crystals can grow and develop their characteristic structures and colours.

How are gemstones formed or created?

Thinking back to geography classes, you might remember terms like sedimentary or igneous, but it may have been a while so let’s have a refresh. Gemstones are formed through various geological processes, namely sedimentary, hydrothermal, igneous and metamorphic formation. Each process creates its own distinct type of gemstone giving it unique properties.

Sedimentary formation

Sedimentary gemstones, as the name suggests, form from sedimentary rocks. They are created first by the accumulation and then the compression of mineral and organic particles. Examples of sedimentary gemstones are:

  • Opals: Formed from silica deposits that fill voids in sedimentary rocks.
  • Ambers: Fossilised tree resin that has hardened over millions of years.
  • Jaspers: Some types of Jaspers are created from the sedimentation of microcrystalline Quartz.
Hydrothermal formation

Hydrothermal gemstones form from high-temperature, mineral-rich water or water vapour that circulates within the Earth's crust. This process can create gemstones such as:

  • Emeralds: Formed when beryllium and other elements are transported by hydrothermal fluids into cavities and fractures within rocks.
  • Aquamarines: Beryllium-rich fluids crystallise within granite pegmatites to form these gemstones.
  • Tourmalines: Created from boron-bearing fluids within the Earth's crust.
Igneous formation

Igneous gemstones crystallise from molten magma or lava deep within the Earth's mantle. It is responsible for some of the most precious stones. Examples include:

  • Diamonds: Form under extreme pressures (about 80,000 bars) and temperatures (around 1,100 degrees Celsius) approximately 140 kilometres below the Earth's surface. They are often found in ancient rocks called cratons.
  • Rubies and Sapphires: Formed from aluminium oxide in igneous rocks like basalt and pegmatite.
Metamorphic formation

Metamorphic gemstones result from the transformation of existing rocks due to extreme high heat and pressure. Examples include:

  • Garnets: Formed when sedimentary or igneous rocks are subjected to intense heat and pressure.
  • Jade: Created from the metamorphism of various rocks, often containing minerals like nephrite or jadeite.

Key formation details of major gemstone varieties

Diamonds

Diamonds are already unique to us in how they appear and what they represent for us culturally, but they are also unique in how they are formed. They come from deep in the Earth’s mantle and under extreme conditions. When carbon deposits are subjected to intense heat and pressure, it causes the crystallisation of carbon atoms into diamond structures. Interestingly, diamonds are often older than the actual rocks they are found in, called cratons, and are brought to the surface by volcanic eruptions.

Emeralds

We have already explained that Emeralds require the presence of beryllium - they are formed when this beryllium in fluid form interacts with surrounding rocks in hydrothermal environments. The enticing green colour of Emeralds is caused due to trace amounts of chromium and sometimes vanadium. They are typically formed in cavities and fractures within the crust and mantle.

Sapphires

Both Sapphires and Rubies are a variety of the mineral corundum. If you remember, they form in igneous rocks and under high-pressure conditions. The shimmering blue of Sapphires is created by the presence of trace elements like iron and titanium, but most Sapphires are treated with heat to enhance these colours and clarity - however, untreated Sapphires are actually more valuable because of how rare they are.

Why it’s important to understand gemstone formation when choosing an engagement ring

The process of gemstone formation is not only fascinating - and a potentially great way to showcase your knowledge of your new engagement ring or item of fine jewellery - but it goes a long way in informing which stone you choose for an engagement or wedding ring. As we now know, how a gemstone is formed affects so many aspects of its quality and appearance, such as colour, clarity, brilliance, and of course, overall beauty. These qualities combine to form how suitable a particular gemstone is for fine jewellery.

Quality and characteristics

The formation process of a gemstone significantly affects its qualities, such as colour, clarity, brilliance, and overall beauty. For instance, Diamonds are renowned for their exceptional hardness and brilliance, making them ideal for daily wear in engagement rings. In contrast, Emeralds, while beautiful, are more brittle and may require protective settings.

Identifying natural, treated and synthetic gemstones

One of the most helpful aspects of knowing the formation process of a gemstone is that it is easier to identify whether it is natural, treated or synthetic. The difference between these is of particular interest if you’re looking for an authentic, untreated gemstone. Earlier we learned that more than 90% of Sapphires undergo heat treatment to enhance their colour and clarity - which makes choosing an untreated Sapphire not only more exclusive, but you can be certain that it is indeed rare. In fact, this treatment is so commonplace that customers aren’t always told.

Tracing geographic origins 

If the ethical sourcing and geographical origins of a gemstone are important to you, then knowing how a gemstone is formed can help trace its origins. However, it doesn’t always tell you exactly where it comes from or how it is extracted. There are many places where gemstones are found in a non-destructive way and have been deposited there from another area. 

For example, deposits in Sri Lanka, known as alluvial gem gravels, consist of gemstones that have eroded and been transported by water over millions of years, and laid to rest in riverbeds. The Namibian coast is known for its Diamond deposits on the seafloor, brought there by water and winds from South Africa and Botswana over millions of years. Crystals like these are usually of exceptional quality, since they have not been broken by the journey or other processes. It also means there are not many inclusions (other materials or gemstones trapped inside, which affects the clarity) so there are fewer defects in the crystal structure.

Durability and wearability considerations

Gemstones form in different ways and those processes create different kinds of crystal structures. This impacts a gemstone’s durability and wearability over time. Two key aspects of this are ‘hardness’ and ‘toughness’, which sound similar but are separate factors in gemstones and both important. 

Hardness means a material’s ability to scratch and be scratched by another, and is measured in the Mohs scale (developed in 1822 by Frederich Mohs). Diamonds are the hardest material known to man and are rated a 10 on the Mohs scale - they can only be scratched by another Diamond. Emeralds are 7.5 on the Mohs scale, which is quite hard, but in contrast they are also brittle and prone to chipping. Toughness therefore refers to a material’s ability to chip or fracture - so Emeralds can be hard, but not as tough as Diamonds. Ivory is also not very hard, but very tough.

Then there is directional hardness and cleavage, which refers to the ease with which a gemstone breaks in specific directions. This affects whether a gem should be faceted or cut cabochon, which is a type of cut whereby the stone is shaped and polished into a smooth, generally convex shape with a flat base.

The romantic and heirloom value of natural gemstones

People choose particular gemstones for many reasons, but almost always there is an emotional connection. The rarity of how a gemstone is created, whether it’s natural, or how it came to look a certain way - all of these factors inform our decision. Understanding and appreciating the intricate details means that not only are we making an informed and wise investment, but also, it enriches the significance and personal connection, making for example a classic white Diamond engagement ring a cherished symbol of commitment. It deepens the romance, meaning and heirloom value of a natural gemstone engagement ring - something as precious and timeless as the ancient forces that forged your favourite gemstone.

There is always more to know and appreciate about authentic gemstones; their history and unique allure are as varied as the people who choose them. We at Blackacre pride ourselves on helping each person make an informed, personal and lasting choice about their fine jewellery purchase, so reach out to us for any guidance, questions or advice.

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