August 21, 2017

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

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.