Gemstones that have a mineral origin are doing in rocks or in gem gravels derived from these rocks. There are five requirement to become gemstones.
The right combination of ingredients, heat, and pressure must last long enough for the minerals to crystallize. Traditionally, we were taught that
There are three kinds of rock formation:
Igneous – Igneous minerals are created with heat. They are minerals that are created deep within the earth.
Metamorphic – Metamorphic refers to conditions where heat and pressure change existing minerals into something new.
Sedimentary – Sedimentary rocks are based on deposits of sediment.
Today, geologists prefer to describe rock formation as four processes:
1. Molten rock & associated fluids
Technically, gems rarely form in the magma itself, but rather from fluids that escape from it (pegmatites and hydrothermal).
2. Environmental changes
Great stresses exist inside the earth. Under the right conditions, the temperature and pressure can rise to the point where existing minerals are no longer stable. Under these conditions, minerals can change into different species without melting.
3. Surface water
Rain plays an important role in recycling minerals. Erosion breaks down rocks and moves them to new locations. Once on the ground, rainwater is instrumental in creating new gems.
As water passes through the earth, it picks up chemicals that turn it into a weak acid. If heated, or mixed with the right chemicals, it can become highly corrosive. That gives water the ability to dissolve even more minerals.
4. Gems formed in the earth’s mantle
Knowledge of the earth’s mantle is still rather limited. However, the evidence shows that some gems actually form in the mantle. To do so, they need to crystallize at an extremely high temperature.
Gems Rising to the Surface After Formation
Since crystals form so far under the surface, you may be wondering how they get to the top where people can mine them. A few crystals are brought to the surface during volcanic eruptions, as described above. However, most reach the surface through mountain building and erosion.
Over vast periods of time, the movement of the continental plates causes mountains to rise. Years of weathering take down the mountain, leaving the deposits near the surface.