It is important to distinguish where gems are formed from where they are found. This rather lengthy article goes some way towards explaining the different ways they can be formed.
Almost all gems are formed below the Earth’s surface; some are brought to the surface through mining and some through natural geological processes such as faulting, folding or volcanic activity. These processes can move rock up from hundreds of miles below the surface, but how are crystals formed in the first place?
Starting at the surface and moving downwards here are the six main ways in which crystals are formed.
1) Formation from water near the Earth’s surface
2) Hydrothermal deposits
4) Magmatic formation
5) Metamorphic formation
6) Gems formed in the mantle
1. Formation From Water Near The Earth’s Surface
These crystals are formed by water near the surface of the earth dissolving minerals and then when the water evaporates then those minerals will precipitate, or form crystals. A good example of this is the formation of salt crystals when sea water evaporates.
The crystal that forms is determined by what the dissolved elements are:
- If the water has interacted with silica (SiO2) rich rocks (e.g. sandstone) then silica-rich minerals will result
e.g. amethyst (quartz); agate ; and the formation of opal.
N.B. Opal is actually not a crystal, it’s really made up of tiny “blobs” of gel less than a micron in diameter.
- If the water has interacted with copper (Cu) rich rocks, copper minerals will form:
e.g. malachite, azurite & turquoise (turquoise requires that the water also picks up some phosphorous along the way)
2. Hydrothermal Deposits
Just the name “Hydrothermal” indicates that water is also involved in this creation process along with heat. Water that is saturated with minerals meets with hot molten rock deep in the earth, combines with further minerals found in the molten rock and is forced along fractures where it eventually begins to cool and these fractures or veins begin to fill with the deposited minerals and crystals. Minerals such as beryl (e.g. emerald),tourmaline need unusual elements, and some of these, like beryllium (for beryl) or boron (for tourmaline) are derived from cooling molten rock (magma)
N.B. One of the most important hydrothermal deposits is the Muzo emerald field in Colombia
Some minerals crystallise more easily than others and pegmatites are formed from the ones which are more reluctant! In the early stages of crystallisation the ions that form high temperature minerals are depleted from the melt and these reluctant minerals become more concentrated. As more minerals crystallise out from the solution the molten rock becomes saturated with water (also silica and unusual elements) and it’s this high water content which allows the crystals to grow quickly – thus forming larger crystal formations or pegmatites.
When the pegmatite magma is rich in beryllium, crystals of beryl form. If magmas are rich in boron, tourmaline will crystallise. You should note that beryllium and boron are extremely rare elements in most rocks and it is only because the above process efficiently concentrates these unusual elements that crystallisation of boron and beryllium-rich minerals can occur.
Some of the world’s best gemstone mines are in pegmatites. Gemstones found in pegmatite include: amazonite, apatite, aquamarine, beryl, emerald, garnet, kunzite, lepidolite, tourmaline and many others.
4. Magmatic Gems
Technically, gems rarely form in the magma itself, but rather from fluids that escape from it, (pegmatites and hydrothermal). The two exceptions to this are called magma and gas crystallisation.
Magma contains a variety of elements. As it cools, the elements combine to form minerals. Exactly what mineral is created varies with the available ingredients, temperature, and pressure. Each time one mineral forms, the available ingredients change. Different minerals form as it goes through the various stages of changing temperature, pressure, and chemistry.
Unless the conditions are just right, crystals will not form. Instead, it will simply cool into a solid mass of small, interlocking crystals; what gemologists call an aggregate e.g. Granite.
In some occasions, one mineral will crystallise nicely. Then, before any more crystals can form, the magma will find a break in the crust and rust towards the surface. Here the pressure and temperature are too low to allow crystallisation. Instead, the rest of the magma cools into fine-grained rocks, with the original crystals distributed throughout the interior. These are called phenocrysts.
Corundum, moonstone, garnet, and zircon are often found as phenocrysts. The Chantaburi and Trat districts in Thailand have large deposits of ruby and sapphire phenocrysts.
Have you ever wondered why some crystals are doubly terminated, where most are broken off at the base? Most crystals grow on a solid base of other minerals. However, a few actually grow inside gas bubbles!
These gems form after the magma has reached the surface. During a volcanic eruption, rising magma undergoes a rapid reduction in pressure. This causes gas bubbles form, just like removing the cork from a bottle of champagne.
Sometimes these bubbles will contain high concentrations of certain elements. If the right combination of temperature and pressure exist for a long enough time, crystals form. garnet, topaz, and spinel also form this way.
One of the best known examples of gas crystallisation are “Herkimer Diamonds,” the water clear quartz crystals from Herkimer, New York.
5. Metamorphic Gems
Metamorphic rocks are rocks changed by heat, pressure, and interaction with solutions. There are a number of types of metamorphic environments:
a) Contact metamorphism occurs when magma forces its way into an existing rock formation. Under the intense heat, existing rocks begin to melt and eventually recrystallize as new species that are stable at higher temperatures. Sri Lanka is one of the best known sites of contact metamorphism. Garnets, corundum, and spinel are also common here. Lapis lazuli, which in found in the mountains of Afghanistan, is another stone created by contact metamorphism.
b) Regional metamorphism takes place on a much broader scale and affects a much greater variety of minerals.
The earth’s surface is composed of large pieces, called “continental plates”. Looking at them from a geological time frame, they are floating on the mantle and in motion. However, they do not all move in the same direction and some of them are actually competing for the same space. Where these huge structures are forced together, one is shoved under and the other is pushed up. This is our primary mountain building method. … Enormous compression forces exist where these land masses come together, creating an area of intense heat and pressure. As the temperature approaches the melting point of rock, the minerals become unstable. Over time, (possibly millions of years,) they change into new varieties.
East Africa is an excellent example of regional metamorphism. Minerals are found here that do not exist anywhere else. Tanzanite is a prime example, as are the unique varieties of garnet.
During metamorphism, some minerals simply change habit. The same ingredients recrystallize in a new crystal system, as a new species. (Remember that a mineral is defined by a combination of its chemical make up and its crystal habit.) These are called polymorphs.
For example, andalusite, kyanite, and sillimanite all have the same chemistry, Al2SiO5. They regularly polymorph by changing into other crystal systems.
6. Gems Formed In The Mantle
The most abundant upper mantle mineral is olivine (peridot). Slabs of mantle material are brought to the surface through tectonic activity and volcanism.
Deep mantle gems. Rocks such as kimberlites are eruptive volcanics that come from quite deep in the mantle and carry with them diamonds. Diamonds are made from carbon. The stable form of carbon at the Earth’s surface is graphite. High pressures and temperatures are required to convert graphite to diamond. Thus, almost all diamonds formed about 100 miles below the Earth’s surface. Dates suggest that their formation was restricted to in the first few billion years of Earth history.
In summary, gems are not always found where they were formed!