Precious stones form during geological processes spanning millions of years.
The three main formation processes
1. Magmatic formation
Gemstones form during the cooling and crystallization of magma. This is the case with diamond, which forms at depths of 150 to 200 km below the Earth's surface, under extreme temperatures (900-1300°C) and considerable pressure.
Examples: Diamond, Peridot, Topaz
2. Metamorphic formation
Some precious stones are born when existing rocks are subjected to high pressures and temperatures, altering their crystalline structure. Emerald forms this way in hydrothermal veins associated with metamorphic rocks.
Examples: Emerald, Ruby, Sapphire, Jade
3. Sedimentary formation
These gemstones form by precipitation of minerals in aqueous environments or by accumulation of organic matter. Turquoise, for example, forms by water infiltration into rocks rich in copper and aluminum.
Examples: Turquoise, Opal, Malachite, Pearl
Determining factors
- ⚗️ Chemical composition: The elements present determine the type of mineral formed
- 🌡️ Temperature: Influences the crystallization speed and crystal size
- 💪 Pressure: Essential for the formation of gemstones like diamond
- ⏱️ Time: Millions of years needed for perfect crystal formation
- 🎨 Impurities: Traces of elements determine color (chromium for ruby, iron for amethyst)
The journey to us
Once formed in depth, gemstones rise to the Earth's surface through tectonic movements, erosion, or volcanic eruptions. Diamonds, for example, are rapidly transported to the surface by explosive kimberlitic eruptions.
Understanding these processes allows gemologists to identify the geographic origin of stones and detect treatments or syntheses.