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

Crystals, Minerals & Rocks

A brief understanding of Crystals, Minerals & Rocks

quartz
Quartz crystal, Madagascar

What are crystals?

Crystals are solid material in which the atoms are arranged in regular geometrical patterns. The crystal shape is the external expression of the mineral’s regular internal atomic structure. Temperature, pressure, chemical conditions and the amount of space available are some of the things that affect their growth. Many crystallise from watery solutions, some from molten rock as during volcanic eruptions when lava cools rapidly.

Each mineral will always form in a range of crystal shapes. Although there are literally thousands of minerals, their crystal shape can be grouped on the basis of their symmetry into seven systems of three dimensional patterns, namely: octagonal, tetragonal, trigonal, hexagonal, orthorhombic, monoclinic and triclinic. (source: gemrock.net)

 

copper mineral
Close up of copper mineral

What are minerals?

Minerals are made of elements. Elements are simple substances that cannot be broken down into any other substance. The name of an element is written down as a combination of letters called a symbol, e.g: sodium is Na; chlorine is Cl. Salt, a combination of sodium and chlorine is thus written as a formula symbol NaCl. Many minerals are made up of large numbers of elements, so their formulas are complex. The most common minerals are those based on silicon and oxygen, Si04.

People value and search for minerals for many different reasons. Most are useful as they are the raw materials of the metals we manufacture into goods. The lead in a pencil is the mineral graphite mixed with clay. Precious metals are used in commerce and other minerals are valued as gems.

Minerals are either found in shapeless lumps which we call ‘massive’ or they can form into the special shapes we recognise as crystals. Most minerals form within the spaces between other minerals and grow into rough shapeless masses. However, if they are able to form freely in a hole or cavity in the surrounding rock the mineral takes the form of a crystal and these crystal lined cavities are called geodes, vugs or pockets. (source: gemrock.net)

 

mushroom tourmaline
Mushroom Tourmaline, Myanmar

What are rocks?

Rocks are combinations of one or more minerals that we find in nature. They can be big or small. There are three types of rocks: igneous (where you find quartz), metamorphic (where you find jadeite), sedimentary (where you find sandstones). Only a few minerals are rock forming and most rock is made from a combination of the commonest of these such as feldspars, quartz, mica, olivine, calcite, pyroxene and amphiboles. Most other minerals, of which there are over 3,000 different types, are rarely present in quantities large enough to be considered rock forming. (source: gemrock.net)

 

If you would like to know more about crystals, minerals and rocks, come and visit The Gem Museum where you will find a whole array of these samples. We look forward to welcoming you!