Igneous rock is formed when magma from the mantle rises up through the Earth’s crust, and cools and solidifies; or when lava on the Earth’s surface cools and solidifies. Igneous rocks are primarily created with heat.
Found in igneous rock: The quartzes (including amethyst, citrine and ametrine), the garnets, moonstone, apatite, diamond, spinel, tanzanite, tourmaline, topaz and zircon.
Metamorphic rock is formed when intense underground heat or pressure alters the existing atomic structures of minerals within rocks, and transforms them into other minerals. In essence, metamorphic rocks are created when heat and pressure change existing minerals into something new.
Found in Metamorphic rock: The beryls (emerald, morganite and aquamarine), jade, lapis lazuli, turquoise, spinel, ruby, sapphire, alexandrite, chrysoberyl and zircon.
Sedimentary rock is formed when rock is worn down and the fragments are carried by water or the wind, and these sediments are finally compressed together over time.
Found in Sedimentary rock: Jasper, malachite, opal and zircon
The Gem Museum presents The Rock Cycle
The Gem Museum is set up to educate visitors on the story of “Mine to Market”. The very beginning of understanding gemstones is to learn about the rock cycle that is a very essential part of the formation of gemstones.
Come and visit us if you are in Singapore! We are opened Monday to Friday, 12pm to 5pm. Admission is free!
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.
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
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.
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.
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
The Andes mountains, formed when the Nazca plate subducted under the South American plate
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.
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.
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