Gems form in many different environments in the Earth.
We will examine the most common and important environments and formation processes. It is important to distinguish where gems are formed from where they are found.
Almost all gems are formed below the Earth's surface.
- Some are brought to the surface through mining
- Some are brought to the surface through earth processes (faulting, folding, large scale uplift, volcanism). These processes can move rock up from more than 400 km below the surface.
In the following article we will examine how gems form. We will start with examples at or near the Earth's surface and move down into the mantle.
1. Formation from water near the Earth's surface
Water near the Earth's surface interacts with minerals and dissolves them. The ability of these solutions to maintain elements in solution varies with physical conditions. If the solution conditions change (for example if the solution cools or evaporates), minerals will precipitate. A similar, familiar processes is formation of salt crystals by evaporation of sea water.
The mineral that forms is determined by what the dissolved elements are. If the water has interacted with silica-rich rocks (e.g., sandstone), silica-rich minerals will form:
- Silica (SiO2)-based minerals: amethyst (quartz); agate ; and the formation of opal. Of these, only opal is non-crystalline (ordered blobs of gel less than a micron in diameter).
- Cu-bearing minerals: malachite and azurite; or turquoise.
The formation of gems by hydrothermal processes is not dissimilar to formation of gems from water near the Earth's surface
The solutions involve rain water and/or water derived from cooling magma bodies< Gems crystallize from solution when it encounters open spaces such as cracks. As a result, 'veins' of minerals fill preexisting cracks.
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).
Pegmatites are unusual magma bodies.
As the main magma body cools, water originally present in low concentrations becomes concentrated in the molten rock because it does not get incorporated into most minerals that crystallize. Consequently, the last, uncrystallized fraction is water rich. It is also rich in other weird elements that also do not like to go into ordinary minerals.
When this water-rich magma (also rich in silica and unusual elements) is expelled in the final stages of crystallization of the magma, it solidifies to form a pegmatite.
The high water content of the magma makes it possible for the crystals to grow quickly, so pegmatite crystals are often large. Of course, this is important for gem specimens!
When the pegmatite magma is rich in beryllium, crystals of beryl form.
If magmas are rich in boron, tourmaline will crystallize.
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 crystallization of boron and beryllium-rich minerals can occur.
4. Magmatic gems
Some gems crystallize in magmas or in gas bubbles (holes) in volcanic rocks. Examples include: zircon, topaz, ruby, etc.
5. Metamorphic gems
Metamorphic rocks are rocks changed by heat, pressure, and interaction with solutions. There are a number of types of metamorphic environments:
- Plate tectonics creates metamorphic environments characterized by high temperature and high pressure - produce jadeite (jade). In extremely rare cases, pressures in metamorphic rocks may be high enough that diamonds form.
- Regionally metamorphosed rocks: large volumes of rock that are buried and changed in response to increases in pressure and temperature. Minerals found in these rocks might include gems such as garnet and cordierite.
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 erruptive 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.
Rarely, diamonds are formed in very high temperature and pressure metamorphic rocks.
Alluvial Gem Deposits:
After rock is brought to the surface, gems may be released from the rock by weathering (some minerals dissolve, others are transformed to clay minerals, and some others survive unchanged). The minerals that survive unchanged may be washed into streams, etc., where they are concentrated by river / ocean processes.
Gems retrieved from alluvial deposits are often rounded due to rolling around in rivers and oceans.
- Gems are often those minerals that are resistant to chemical weathering. They are commonly concentrated in stream beds and beach sands in what are known as alluvial deposits.
- Gems often have quite a high specific gravity (density) compared to other minerals so that they are easily trapped in depressions in stream beds. This causes them to become concentrated and makes it easier to mine them. Other valuable and durable things are also concentrated by these processes. Gold is a well known example.
- In summary, gems are not always found where they were formed, nor are they formed where they're found!
Material Provided by Berkeley University of California.