There are five requirements for crystals to form: ingredients and temperature, pressure, time, and space. To better explain the basics of mineral crystallization lets talk about rock candy for a second. Rock candy is simply crystallised sugar.
If you stir as much sugar as possible into a bottom of water you will see it begin to settle at the bottom of the pot. When no more sugar can dissolve, you have reached saturation point. The water can’t absorb any more sugar, this is known as super-saturated.
Now bring the pot to a boil- the saturation level changes at boiling point. You can add more sugar, you should do so until you reach super-saturation. At this stage, you should remove the pot from heat. As the water returns to room temperature the sugar it can hold returns to its previous level. The excess sugar comes out of its solution and crystallises as it does so.
Now, hang a string in the solution so the crystals have something to grow on. Ideally, weight the string to keep it straight. Once the water has totally cooled the string will be covered in crystals.
This is a great way to understand just how gemstones form.
Gemstone Formation Process
Generally speaking there are 4 ways gemstones can form. They are
- Igneous– These minerals are created deep within the earth (Diamonds, Ruby, Sapphire, Peridot)
- Hydrothermal– Similar to the rock candy example, gemstones are formed when bodies of mineral rich water cool
- Metamorphic– As the name suggests, these are gems that are ‘morphed’ due to intense heat and pressure. (Sapphire, Ruby, Spinel, Garnet)
- Sedimentary– Gems that form due to water depositing sediments (Malachite, Azurite, Opal)
Igneous Gemstones Formed in the Earth’s Mantle
While our knowledge of the earth’s mantle is limited, there is evidence that some gemstones form in the mantle. This requires extremely high temperatures.
Perhaps the most notable examples of gemstones forming in the earth’s mantle are peridot and diamond. Geologists studied Arizonan peridot deposits and believe that they were created on rocks that were floating in the earth’s mantle, up to 55 miles beneath the surface. They were brought closer to the surface by an explosive eruption, with erosion and weathering pushing them close enough to the surface to be discovered.
However, there is a better understanding of diamonds. Diamonds crystallise in the magma just below the crust. However, these formations have a different chemical composition. Geologists believe it comes from 110 miles to 150 miles beneath the earth’s surface. The magma is incredibly fluid at this depth, and the temperatures very high.
This magma can force its way through the crust far faster and much more violently than other volcanic eruptions. During the process of eruption, the magma breaks up and dissolves rocks and then carries them to the surface.
If the magma was to rise slowly the diamonds likely would not survive. The pressure and changing temperatures would result in the diamonds vaporising, or possibly re-crystallising as graphite. However, because of the speed with which the magma rises the diamonds don’t have time to transform or vaporise, thus remaining as diamonds.
When there are dramatic and rough changes in the crust crystals are often broken. When growth conditions are present material seeps into the fractures and crystallises. This heals the fractures by sealing them together. They don’t heal completely, though, the fine cavities remain and they are seen as fingerprints.
How do they rise to the surface once gemstones form? Because they form so far under the surface, it’s a wonder they can be mined. They’re brought to the surface during volcanic eruptions, but most of them reach the surface through erosion and mountain building.
Hydrothermal Gemstone Creation
This process is the most similar to the rock candy given above. Supersaturated water with many different minerals is pushed up into cavities and cracks in the earth. as this solution begins to cool the different minerals begin to crystallize.
The most important hydrothermal finds are in Columbia. Specifically the Muzo Emerald mine. These hydrothermal deposits are rich in Chromium with gives the Emeralds from the region their incredible color.
The image below shows a hydrothermal mineral vein. This vein is created when the water solution cools inside the crack of the surrounding rock.
Metamorphic Gemstone Creation
The majority of gemstones are formed by metamorphism. This is when minerals are forced together under great pressure and heat usually by tectonic plates moving underneath each other. The minerals are forced together and they metamorphose into different minerals, sometimes without melting.
Sedimentary Gemstone Creation
Sedimentary gemstone creation occurs when water mixes with minerals on the earth’s surface. The mineral-rich water seeps down between the cracks and cavities in the earth and deposits layers of minerals. This is how minerals like Opal, Malachite, and Azurite are formed. Opal is formed when the water mixes with silica. As the silica solution settles, microscopic spheres of silica stack on top of each other forming Opal.
The earth’s crust can be anywhere from three miles thick to 25 miles thick. Beneath the crust is the earth’s mantle. The mantle is approximately 1,860 miles thick and makes up 83% of the earth’s volume. It is composed of magma, which is molten rock. When it reaches the surface, it is called lava. It is at its hottest nearer the center of the earth you get- and the currents of heat keep it in constant motion.
The zone where the crust and mantle meet is tumultuous, with high temperatures and high pressures. Several plates make up the earth’s crust and float on the liquid mantle. As they run into each other some are raised into mountains, while others are pushed down.
Magma is also in continual motion. Its pressure and movement are continuously creating wear and fractures to the bottom of the crust. Rocks then break free from the earth’s crust, being carried away by the fluid magma. The rocks melt, altering the magma’s chemistry. While the smaller particles are fated to become inclusions in gemstones yet to form.
Gemstones form deep in the earth, with the lower surface of its crust containing numerous cavities due to heavy fractures. Fluids escape through the cavities and fractures. This is the ideal condition for crystal growth. It is essentially a soup rich in chemicals, supplying all of the necessary ingredients. The cavities provide the perfect space to grow, and the pressure and temperature are high. The fluid moving through the crust causes it to cool enough for the crystallization to ensue- all that’s needed now is time.
Geologically speaking- the time it has should be sufficient. However, because this environment is highly tumultuous and the passages continuously open and collapse. The crystals will often start to form, but when the passage collapses the fluid stream is closed off. It’s at this point that growth stops.
If and when the passage reopens the growth resumes. This on/off growth is generally undetectable in crystals, though in other cases the successive layers of development have a different chemical composition. This results in color zoning.
Mineral Crystallisation Order
Topaz crystals form before quartz in the cooling process because one principle of crystallization is that as the temperature drops the solid ingredients it can hold drops. Though, the ingredients in the earth’s crust are a tad more complex than the sugar solution described above. Different minerals crystallize from the same solution, but at different temperatures. You may see corundum first, followed by topaz and quartz as the solution continues the cooling process.
While pressure has no effect on rock candy- the correct combination of temperature and pressure is needed for minerals to crystallize.
Additionally, there are two other conditions required for crystallization – space and time. Essentially, the correct ingredient combination, pressure, and heat must last long enough for minerals to crystallize. Additionally, they require room to grow.
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How Are Gemstones Created?
How to Create Your Own Crystals?
Making crystals is an excellent way to learn about crystal growth. To make rock candy, the simplest method is to crystallize the sugar.
- Add as much sugar as possible to a pot of water. When it reaches the bottom and no longer dissolves, you have reached the saturation point. The water has absorbed all of the available sugar. This is referred to as super-saturation.
- Following that, bring the pot to a boil. When water boils, the saturation point changes. The solution is no longer super-saturated. You can now significantly increase the sugar content. Therefore, continue adding sugar until you reach a super-saturation level.
- Take the pot off the heat. When the water reaches room temperature, the amount of sugar suspended in it returns to its previous level. Excess sugar must be removed from the solution. It will crystallize as it does so.
- In the sugar solution, hang a string for the crystals to grow on. (At the bottom of the string, fix it with a weight to keep it straight.) Although the process is too slow to observe without assistance, you will notice changes in the crystals every few minutes.
- When the solution reaches room temperature, the string will be completely covered in sugar crystals. Again, the water will be supersaturated.
The Fundamentals of Mineral Crystallization
This straightforward exercise introduces you to four of the five conditions necessary for mineral crystallization to occur.
Crystals are composed of more complex and numerous ingredients than our sugar solution. Various minerals may be present in solutions.
At a greater temperature, a solution can suspend a large number of minerals. As the temperature drops, the amount of suspended solids decreases as well. Crystals form as a result of this. Indeed, different minerals crystallize at different temperatures in the same solution. Corundum, for example, may crystallize first. As the solution cools further, topaz and then quartz may form.
There is no effect of pressure on the formation of rock candy. However, minerals crystallize only under the right conditions of pressure and temperature. Underground crystallization of gems typically requires extremely high pressures and temperatures.
Time and space are relatively simple requirements. The optimal combination of ingredients, heat, and pressure must exist for an extended period of time in order for the minerals to crystallize. Additionally, they require room to grow. Obviously, a crystal 5 cm in length cannot grow in a cavity only 5 mm in diameter.
Consider the underground conditions that permit crystallization and the formation of gems.
The thickness of the Earth’s crust varies from 3 miles (around 4.8 kilometers) beneath the seabed to 25 miles (around 40 kilometers) beneath continents.
The mantle is approximately 1,860 miles (around 2993 kilometers) thick beneath the crust and accounts for 83% of the Earth’s volume.
It is made up of molten rock known as magma. When it does reach the surface, we refer to it as lava.
Near the Earth’s center, the mantle is hottest, and heat currents keep it in constant motion.
The crust and mantle cross paths in a violent zone characterized by high pressures and temperatures.
The crust is composed of several plates that float on the liquid mantle. As they collide, some are pushed down and others are elevated into mountains.
Additionally, the magma is in constant motion. Its constant movement and pressure act on the crust’s bottom, causing wear and fracturing.
Its constant movement and pressure act on the crust’s bottom, causing wear and fracturing. As a result, rocks separate from the crust and are carried away by the magma’s fluidity.
Much of this rock material melts, altering the chemical composition of the magma nearby.
Several of the smaller particles will be used as inclusions in future gemstones.
The lower surface of the crust is heavily fractured and contains numerous cavities.
The magma’s fluids flow through these fractures and cavities. Here, the ideal conditions for crystal growth exist.
Chemically rich fluids provide the necessary components. Cavities provide space for growth.
The temperature and pressure are extremely high in this area. The fluid cools sufficiently as it passes through the crust, allowing crystallization to occur. The only remaining requirement is time.
How Crystal Growth Interruptions Affect Gem Formation?
You might believe that time is more than sufficient for crystal formation in geological terms.
However, passages constantly open and close in this highly turbulent environment.
Often, crystals begin to form and then the passageway feeding the cavity with mineral-rich fluid closes.
At this point, no further growth occurs.
Growth will resume if the passage is reopened.
Growth will resume if the passage is reopened. This on-and-off growth pattern is usually undetectable in crystals. However, it does have noticeable effects in some instances.
Zoning by Color
Occasionally, the chemical compositions of successive layers of growth will be slightly different. When this occurs, the crystal may exhibit color zoning.
In some twinned crystals, the new layers do not completely bond to one another. When you see parting on a star ruby, for example, this indicates that the layers did not bond properly.
Crystallized Mineral Specimens
Even if a closed passage is reopened and fluid is allowed to enter a cavity again, a completely different mineral may crystallize over the previously crystallized material. Indeed, the temperature, pressure, and chemical composition of a fluid solution frequently change over time. Within a cavity, different conditions will result in the formation of different mineral crystals. When you open a deposit, you will frequently notice that different minerals cover earlier layers.
Such changes in the internal conditions of a cavity are also one of the causes of gemstone inclusions. A new crystal may begin to grow on the surface of an older, larger one, only to have its growth stopped. If the original crystal-growing conditions are restored, the older crystal will grow over the newer one.
Occasionally, two distinct minerals crystallize simultaneously. If one takes off and begins to grow at a faster rate, it will eventually entrap the other. That is how pyrite crystals become embedded in emeralds.
Chemical impurities can also exist within a crystal under certain conditions. When the temperature and/or pressure of the host crystal change, the impurities can crystallize within it. (In effect, the host crystal serves as a cavity that contains ingredients that require only the right conditions to crystallize.) This is the process by which rutile forms within quartz and corundum.
Phantom crystals or inclusions may form in rare instances. This occurs when a new layer of crystals forms on top of an existing transparent crystal. A fine layer of feldspar, for example, may cover a quartz crystal. Subsequently, conditions revert to their original state, and the original transparent crystal resumes growth. This time, the feldspar is covered by a new layer of quartz. The resulting gemstone exhibits an indistinct outline of that fine secondary crystal layer, resembling a nearly transparent ghost or phantom, hence the name.
Numerous crystals are broken during the rough and dramatic changes in the crust. However, if growth conditions exist, material may seep into fractures and crystallize. This effectively “heals” the fracture by regrowing the crystal. However, these fractures never completely heal. In the previous gap, fine cavities filled with gas remain. This type of inclusion is referred to as a healing fracture. Due to the similarity between the fine cavities that remain and fingerprints, gemologists also refer to these inclusions as “fingerprints.”
Numerous crystals are compressed beyond their natural size due to the tremendous pressures present in the underground environment of gem formation. Additionally, this strain can make a stone more brittle. This level of strain may be present in tourmalines, garnets, and even diamonds. Numerous faceters have shattered these stones when they were placed on a lap. The forces within the stone literally explode it.
Geological Processes and the Formation of Gemstones
Gemologists now have a good understanding of mineral crystallization. Advances in geology and synthetic gem manufacturing have aided in the unraveling of several of nature’s mysteries. (Creating gems in laboratories involves simulating underground conditions, but significantly reducing the time required.)
Traditionally, we were taught that there are three distinct processes that result in the formation of rocks:
Igneous rocks are formed by intense heat deep in the Earth.
Metamorphic rocks are formed when existing minerals undergo transformation due to heat and pressure.
Sedimentary rocks are formed when sediment is deposited.
Geologists now prefer to describe rock formation in terms of four distinct processes:
Molten rock and fluids associated with it
Changes in the environment
Water on the surface
Formation of gems in the Earth’s mantle
We will look at how each of these processes contributes to the formation of gems.
Regardless of our knowledge, mineral and gemstone formation are neither simple nor straightforward. Minerals and gems are constantly destroyed and recreated as a result of the “Rock Cycle,” as illustrated in the chart below.
Molten Rock and Associated Fluids
Gems rarely form in the Earth’s magma. Rather than that, they form as a result of fluids escaping from it, as with gems from hydrothermal deposits and pegmatites. To begin, we’ll discuss two notable exceptions to this rule: magma and gas crystallization.
Crystallization of Magma
Magma is composed of numerous elements. The elements combine to form minerals as it cools. Which mineral is used depends on the available ingredients, the temperature, and the pressure. Each time a mineral crystallizes, the ingredients that are available change (since some ingredients are pulled into crystals). Different minerals form as magma progresses through various stages of changing temperature, pressure, and chemistry.
However, crystals will not form unless the conditions are ideal. Rather than that, magma will simply cool into a solid mass of small, interlocking crystals referred to as an aggregate by gemologists.
In some instances, a single mineral crystallizes beautifully. The magma will then seek out a crack in the crust and rush to the surface, destroying any remaining crystals. The pressure and temperature are too low for crystallization to occur. Rather than that, the remaining magma cools into fine-grained rocks, leaving the original crystals scattered throughout the interior. These are referred to as phenocrysts.
Phenocrysts of corundum, moonstone, garnet, and zircon are frequently discovered. Thailand’s Chanthaburi and Trat districts are rich in ruby and sapphire phenocryst deposits.
The Crystallization of Diamond
Diamonds crystallize at significantly higher temperatures than other minerals. Scientists now believe that the majority of diamonds form in magma, close to the Earth’s crust, where the temperature is the coolest. If this is true, it also means that the most favorable conditions for diamond crystallization exist underground. Diamonds are possibly the most abundant crystals on the planet. They are simply not the most accessible.
Crystallization of Gas
Have you ever wondered why some crystals are doubly-terminated, while the majority of crystals are broken at the base? The majority of crystals form on a solid matrix of other minerals. However, a few thrive within gas bubbles! After the magma reaches the surface, these gems form. During a volcanic eruption, rising magma’s pressure rapidly decreases. This results in the formation of gas bubbles, much like when removing the cork from a bottle of champagne.
Occasionally, these bubbles will contain elevated concentrations of particular elements. If the proper temperature and pressure conditions exist for an extended period of time, doubly terminated crystals form.
The so-called “Herkimer diamonds” are some of the most well-known examples of gas crystallization. The first half of the name of these quartz crystals comes from their source in Herkimer, New York. To some gem enthusiasts, these gems’ water-clear clarity and shape evoke the appearance of diamonds, hence the second half of their nickname. (Obviously, quartz gemstones are not diamonds.)
Garnet, topaz, and spinel can also crystallize as a result of gas crystallization.
Deposits of Hydrothermal
Hydrothermal crystallization, as the name implies, involves the use of water and heat. Water dissolves minerals as it percolates through the Earth. (Exactly as the sugar in your rock candy did). It comes into contact with magma deep within the Earth. The magma then spews out special fluids containing water, carbon dioxide, and volatiles (substances that give off the gas).
These hydrothermal fluids flow through crustal fractures. They may dissolve minerals or combine with other groundwaters along the way. Mineral-rich fluids cool in “veins.” Crystals form when the right conditions of temperature, pressure, time, and space are met.
Hydrothermal deposits are unique in that they may contain elemental combinations not found elsewhere. The Muzo emerald field in Colombia is one of the most significant hydrothermal gemstone deposits.
At times, the magma in the upper mantle becomes concentrated with volatiles. Occasionally, this volatile-rich magma is forced into a cavity, where it cools and crystallizes into a pegmatite. Pegmatites are distinct from hydrothermal veins in that they are formed by magma rather than water.
Changes in the Environment
Within the Earth, extremely large pressure exists. Temperatures and pressures can rise to a point where existing minerals cannot remain stable under the right conditions. This can result in the transformation of minerals into new species without melting. This is referred to as metamorphism.
Metamorphism is classified into two types: contact and regional.
Make Contact with Metamorphism
When magma forces its way into an existing rock formation, this is called contact metamorphism. Existing rocks begin to melt and eventually recrystallize as new species when exposed to extreme heat. These are thermally stable at elevated temperatures.
Regional metamorphism is much more widespread than contact metamorphism. It has a much broader range of mineral effects.
The Earth’s surface is made up of large pieces called “continental plates.” They float on the mantle and are constantly moving on a geological time scale. They do not, however, all move in the same direction. Several of them are even in direct competition for the same space. Where these massive structures are pressed together, one is pushed beneath the other, while the other is pushed upward. This is the primary method of mountain formation on our planet.
Where these landmasses meet, enormous compression forces exist. This results in the formation of an area of extreme heat and pressure. When the temperature in the area approaches the melting point of the rock, the minerals become unstable. They evolve into new species over time (possibly millions of years).
East Africa is a prime example of a region undergoing regional metamorphism. Certain minerals discovered here are unique, such as tanzanite and commercial-quality tsavorite.
The Development of Mineral Species
To comprehend some of the changes that occur to minerals during metamorphism, it is necessary to understand what constitutes a mineral species: both its chemical formula and crystal structure. Occasionally, during metamorphism, a mineral change only one of these properties. This is still considered a species change. These changes result in the formation of polymorphic and pseudomorphic species.
Polymorphs are minerals that share the same chemistry but have distinct crystal structures. Mineral pairs that are polymorphous are occasionally referred to as dimorphs or dimorphous.
For instance, andalusite, kyanite, and sillimanite are all composed of the same chemical compound, Al2SiO5. They frequently polymorph into one another during metamorphism, when their chemical constituents recrystallize into new crystal structures. As a result, they evolve into distinct but polymorphous species.
Pseudomorphs are minerals that undergo chemical transformations without altering their external crystal structure.
While some minerals undergo metamorphism, they retain their characteristic crystal structures and exhibit no abnormal properties. However, a crystal’s chemistry can change without recrystallization. Pseudomorphs are these one-of-a-kind minerals. A pseudomorph is an atomic-by-atomic substitution of one mineral for another without altering the outward shape of the original mineral.
A perfect example of a pseudomorph is the tiger’s eye. Quartz has replaced the original crocidolite (a type of asbestos) in these gems, but they retain the crocidolite’s fibrous structure.
Marcasite can pseudomorph into other minerals such as pyrite, gypsum, fluorite, and goethite.
Azurite frequently pseudomorphs into malachite, resulting in an azurite crystal with the perfect malachite crystal shape.
When a mineral forms a pseudomorph, it is referred to as the “after” mineral. Thus, we may encounter “pyrite following marcasite,” “gypsum following marcasite,” “malachite following azurite,” and so forth.
Water on the Surface
Rain is critical for mineral recycling. Erosion erodes rocks and repositions them. Once on the ground, rainwater plays a critical role in the formation of new gems.
Formation of Fossils
Water picks up chemicals as it travels through the Earth, transforming it into a weak acid. When heated or when combined with the appropriate chemicals, it can become extremely corrosive. This increases the ability of water to dissolve additional minerals. As water percolates through the Earth, it picks up a variety of substances. When it becomes too saturated to carry any additional material, it deposits the excess in the cracks and pores of existing rocks. This is the process by which fossils and petrified wood are formed.
In other circumstances, water comes into contact with mineral combinations that initiate a chemical reaction. The dissolved minerals are then deposited in seams and cavities as new minerals. Opal, turquoise, azurite, and malachite are all formed in this manner.
Formation of Opals
Much of central Australia was covered by an inland sea during the Cretaceous period. When it dried, it left a layer of silica-rich sands in the area. Rain has been dissolving silica for millions of years. Summers are hot and dry, and the groundwater evaporates to the point where the remaining water is unable to suspend the silica. Excess moisture is deposited in seams and cavities near the surface. Opal is a silica deposit.
Waterborne Minerals Can Create Gem Color
Certain gemstones receive their color primarily from minerals that are necessary for their chemical composition. Turquoise, azurite, and malachite, for example, all receive their color from copper dissolved in water. To form azurite or malachite, copper-rich water must pass through limestone. Turquoise also requires some phosphorous to be added to the water along the way.
Gems Formed in the Mantle of the Earth
Our understanding of the Earth’s mantle remains rather limited. However, evidence indicates that certain gems do form in the mantle. They must crystallize at an extremely high temperature in order to do so. Diamond and peridot are two of the most well-known gems formed in the Earth’s mantle. (Intriguingly, both of these gems can be found extraterrestrially. Both of these elements have been discovered in meteorites).
Formation of the Peridot
Geologists now believe that some peridots were formed on rocks floating in the mantle, approximately 20 to 55 miles (32 to 89 kilometers) below the surface. They were brought close to the Earth’s surface by an explosive eruption. Weathering and erosion eventually brought them to the surface, where they were discovered.
Formation of a Diamond
The formation of diamonds is now better understood. As previously stated, the majority of diamonds crystallize in the magma beneath the crust. However, the magma formations in which they are found are chemically distinct. They may originate from greater depths, approximately 110 to 150 miles (177 to 241 kilometers) below the surface. Temperatures are higher and the magma is very fluid at this depth.
This hot and fluid magma can pierce the crust more quickly and violently than other types of volcanic eruptions. It will disintegrate and dissolve lower mantle rocks during this process and then transport them to the surface.
If the magma rose more slowly, the diamonds would almost certainly perish. They would vaporize or recrystallize as graphite as a result of the changing temperatures and pressures. The magma may rise at such a rapid rate that the diamonds do not have time to transform.
How Mountain Formation and Erosion Bring Gemstones to the Surface?
As previously stated, a few types of gems are exposed during volcanic eruptions. What about the countless others that crystallize deep underground? The majority of these gems reach the surface as a result of mountain formation and erosion. Mountains rise as a result of the movement of the continental plates over long periods of time. The mountain is then weathered away over time, leaving the gem deposits near the surface. Naturally, this process takes hundreds of millions of years.
Article Composed by :
Chairman: Gemological Institute of Ceylon
Chairman: Youth Gem Professionals Association
Chairman: Sampath Gems
Director: Ceylon Sapphire Gems and Jewels
Director: Ceylon Gem Fair International
Chairman: Nanosoft Web Develop Company
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