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How do minerals form underground?
Answer 1:

Almost all minerals and 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 like faulting, folding, or volcanic eruptions.

How do minerals and gems form?
Water near the Earth's surface interacts with minerals and dissolves them. If the solution cools or evaporates, minerals will precipitate. A similar, familiar process is formation of salt crystals by evaporation of seawater.
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 like Silica (SiO2)-based minerals: amethyst (quartz); or opal. Of these, only opal is non-crystalline (ordered blobs of gel less than a micron in diameter).
If the water has interacted with copper-rich rocks, copper minerals will form:
Cu-bearing minerals: malachite and azurite or turquoise.

Another way is the formation of gems by hydrothermal processes The solutions involve rain water and/or water derived from cooling magma . Gems crystallize from solution when it encounters open spaces such as cracks. As a result, 'veins' of minerals fill preexisting cracks.
Minerals such as emeralds or tourmaline need unusual elements, like beryllium (for emeralds) or boron (for tourmaline)

Metamorphic gemsMetamorphic rocks are rocks changed by heat, pressure, and interaction with solutions.
This high temperature and high pressure is needed to produce jadeite (jade). In extremely rare cases, pressures in metamorphic rocks may be high enough that diamonds form.

Some gems are formed in the mantle.The most abundant upper mantle mineral is olivine. Slabs of mantle material are brought to the surface through tectonic activity and volcanism.
Gems that form deep in the mantle :
Rocks such as kimberlites 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.

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