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).
4. Magmatic gems
5. Metamorphic gems
- 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.
6. Gems formed in the mantle
- 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!
The above story is based on Material Provided by Berkeley University of California.