Gemstones that form in the Earth’s mantle

Only two gemstones form in the Earth’s mantle

diamonds can be found in the earth's mantle

Yellow Diamond from South Africa, a country known for its diamond mines. 

(Photo by Eric Nathan/Alamy, taken from National Geographic)

 

Together with diamonds, peridots are also found in the earth's mantleOlivine (Peridot) at the Natural History Museum, London.

(Photo by Aram Dulyan, via Wikimedia Commons, taken from International Gem Society)

 

Most gemstones form in the Earth’s crust. Only two gemstones form in the Earth’s mantle – the diamond and the peridot. These are gems that crystallise at extremely high temperatures.

 Fun fact: Diamonds may actually be the most plentiful crystals in the earth, but they just aren’t the easiest to reach

Diamonds are formed in the earth’s mantle.

Diamonds crystallise in very hot and fluid magma about 110 to 150 miles (117 to 241 kilometres) below the Earth’s surface. Because of the heat and fluidity, this magma is able to force a pathway up through the Earth’s crust at greater speeds and violence than other volcanic eruptions! These are called “kimberlite pipes”.

A simplified diagram of kimberlite pipe in earth's mantle and surface

  1. Magma pocket comes in contact with a weak area in crust.
  2. A quick explosion results, carrying diamond-bearing magma from the lower mantle to the surface. During the eruption, a cone builds on surface.
  3. The pipe eventually cools, leaving carrot shaped pipe.
  4. The cone quickly erodes away, leaving the diamond-bearing rocks where people can reach them.

(Diagram and description taken from International Gem Society)

 

If the rising of magma were any slower, the diamonds would likely not survive, as the changing temperatures and pressure would likely have caused them to vaporise or recrystallise as graphite.

This video shows a simple set up at The Gem Museum. It shows the formation and mining of diamonds in the world.

Peridot

Geologists believe that peridot first crystallises on rocks floating in the mantle about 20 to 25 miles (32 to 89 kilometres) below the Earth’s surface. Then an explosive eruption brings them near the Earth’s surface, and weathering and erosion brings them close enough to be mined and found.

 

Earth's mantle forms peridots
Peridot crystal from Pakistan (The Gem Museum)

 

peridot forms in earth's mantle
Peridot crystals from Lisu Village, North Myanmar. (The Gem Museum)

References / Further reading:

Gem formation – International Gem Society

Where did those gemstones come from – ThermoFisher Scientific

Gemstone Formation in Earth’s Crust

Gemstone in Earth’s Crust

gemstones
A raw crystal held up to the sunlight, Spinel surrounded by corundum (Vietnam).

 

 

Twin Spinel gemstone
Looking at a piece of twin spinel, Northern Vietnam.

 

  → 

 

Alexandrite

LEFT: Alexandrite cut and polished. | RIGHT: Alexandrite in the rough.

(Photos from Gemological Institute of America)

 


→ 

Sapphire

LEFT: Sapphire cut and polished. | RIGHT: Sapphire in the rough.

(Photos from Gemological Institute of America)

 

 

emerald gemstone → 

Emerald

LEFT: Emerald cut and polished. | RIGHT: Emerald in the rough.

(Photos from Gemological Institute of America)

 

 

 → 

Ruby

LEFT: Ruby cut and polished. | RIGHT: Ruby in the rough.

(Photos from Gemological Institute of America)

 

Coco Chanel wanted to “cover women with constellations”. Like stars that pattern the night with light, jewels adorn the woman. But before being fashioned into jewellery, a gemstone is usually first a mineral (a naturally-occurring, inorganic solid with a definite chemical composition and an ordered atomic arrangement) formed by geological processes in the Earth’s crust.

 

Emeralds, for instance, are formed of the mineral Beryl, a chemical compound containing beryllium, aluminium, silicon and oxygen. Its colour comes from additional traces of chromium and vanadium, but different trace elements will produce different colours, allowing the mineral Beryl to form semi-precious stones such as Aquamarine.  

 

Rubies are formed of the mineral Corundum, and its red colour comes from traces of chromium. Sapphires are also formed of Corundum, and can come in different colours depending on its trace mixtures of iron, titanium, and chromium.

 

A single rock can be composed of different minerals. From rocks, mineral crystals are mined, cut and polished into precious or semiprecious gems.

 

How do minerals form in the Earth?

The International Gem Society has a very comprehensive article on how minerals form. In summary, minerals crystallise when conditions of temperature (heat), pressure and time to grow are right, and when there is space to grow.

 

Heat and Pressure determine the type of mineral formed.

Some minerals (such as Quartz – below left) require very little heat and pressure to form, and will melt or break down under great heat and pressure.

Other minerals (such as Pyrite– below right) need a lot of heat and pressure to form, because at low temperatures, its raw materials can stay mixed up with other minerals’ raw materials.

(Photo from American Museum of Natural History)

 

How large a crystal you get is affected by Time.

The more time there is for the crystal to grow, the larger it will be, because minerals have an ordered atomic arrangement and it takes time for atoms to be ordered. Since ions are more mobile in water, the presence of water can help accelerate the mineral growing process.

 

The earth is like a beautiful bride who needs no manmade jewels to heighten her loveliness.  – Kahlil Gibran

 

(Rock Cycle Diagram from The Geological Society)

There are 4 main processes by which gems form

(Information from the International Gem Society):

  • Molten rock & associated fluids
  • Metamorphism
  • Surface water
  • Gems formed in the earth’s mantle

 

Molten rock & associated fluids

(Magma crystallisation, Gas crystallisation, Hydrothermal, Pegmatites)

Lava spews out of a fissure in the Virunga mountains

(Photo by Chris Johns, taken from National Geographic)

A river of molten lava

(Photo by Carsten Peter, taken from National Geographic)

 

Magma crystallisation

Most minerals found in the rocks around us are formed when molten rock (or magma) rises up through the Earth’s crust due to volcanic eruptions or by heat currents that keep the magma in constant motion.

 

As the hot magma cools, minerals crystallise.

    • If the cooling process is extremely rapid (seconds), no crystals will form, resulting in volcanic glass (not composed of minerals). However, over millions of years, the volcanic glass may crystallise.
    • If the cooling process is rapid (minutes, hours, days, or years), the components of the minerals will not have time to be ordered, and the crystals formed will be small (less than 1mm), resulting in a fine-grained rock.
    • If the cooling process is slow (decades to millions of years), the crystals formed will be large.

 

During a volcanic eruption, when there is a rapid drop in pressure, gas bubbles can form in the rising magma (Imagine popping a cork from a champagne bottle.) Sometimes these gas bubbles contain a high concentration of certain elements. If the right conditions of heat, pressure and time exist, minerals will crystallise.

 

Hydrothermal

When water (e.g. Rain) seeps down through the soil and meets with rising magma, hydrothermal fluids are formed. Such hydrothermal fluids continue flowing through fractures in the Earth’s crust, dissolving minerals along the way. When these mineral-rich hydrothermal fluids cool in “veins” and the right conditions of heat, pressure and time exist, minerals will crystallise.

 

Pegamatites

When volatile-rich magma cools in cavities and the right conditions of heat, pressure and time exist, minerals will crystallise.

 

Metamorphism – when minerals form from other minerals

(Contact, Regional)

The Andes mountains, formed when the Nazca plate subducted under the South American plate

(Photo by Peter Essick, taken from National Geographic)

 

Contact metamorphism

When magma forces its way into an existing rock formation, the intense heat breaks down existing rocks and causes minerals to re-crystallise into more stable forms.

 

Regional metamorphism

Intense heat and pressure arises when tectonic plates below the Earth are shoved toward each other, forcing one on top of the other (This is how mountains form). As the temperature approaches the melting point of the rock, the minerals become unstable. Over millions of years, they change into new varieties.

 

Surface water

When rainwater flows through the Earth, it deposits the minerals that are dissolved in it along the way into cracks in the existing rocks. Such deposits then lead to the formation of petrified wood, fossils and new minerals.

 

Gems formed in the Earth’s mantle

Further underground below the Earth’s crust is the Earth’s mantle. While most gemstones form in the Earth’s crust, some gems do form in the mantle, amongst which are diamonds and peridots. Read more about how they form here.

Colourful mineral deposits in the Earth’s landscape

(Photo by Thinkstock/Corbis, taken from National Geographic)

 

Gypsum formations that mimic flowers found in Mammoth Cave in Kentucky

(The gypsum precipitates out of water in the cave, creating these subterranean forms over time)

(Photo by Tom Uhlman/Alamy, taken from National Geographic)

 

References / Further reading:

Story of a gem – Gemological institute of America

What is a gem? – University of Berkeley

What is a mineral? – Oxford University Museum of Natural History

Minerals and Gems – National Geographic

Mineral forming environments – American Museum of Natural History

Formation of minerals – BC open textbooks

FAQ Mineral formation – University of California

Gem formation – International Gem Society

Plate tectonics — National Geographic