What Gems Are Found in Igneous Rock

Gemstones found in igneous rock include the quartzes (including amethyst, citrine and ametrine), the garnets, moonstone, apatite, diamond, spinel, tanzanite, tourmaline, topaz and zircon. Some of these gemstones form in pegmatites and hydrothermal veins that are genetically related to igneous rocks.

By the cooling down of magma, atoms are linked into crystalline patterns and subsequently different minerals are formed. When the formation takes place in the depths of the earth's crust (approx. 33km deep) quite large rocks may be formed (for instance, granites).

Igneous rocks are formed and created by magmatic processes in the earth. To form very large crystals of rare minerals, exceptional conditions are needed. For instance, a rock called pegmatite is formed by the crystallization of magma enriched with water in the veins of other rocks, and may contain beryl, tourmaline and topaz.

What Gems Are Found in Igneous Rock?
What Gems Are Found in Igneous Rock.
Photo from The Rock and Gem Book: And Other Treasures of the Natural World

Igneous rocks are divided into two types -- volcanic rock (extrusive) and plutonic rock (intrusive) -- depending on where the magma cools.

Volcanic or extrusive rock

This is rock that is formed on the surface of the earth. In contact with air or seawater, molten rock cools rapidly and either quenches to a glass (like obsidian) or forms small crystals (basalt). Volcanic rocks are usually finely grained or glassy in structure.

 Basalt is an extrusive rock, finely grained due to its rapid cooling. It largely consists of tiny feldspar and pyroxene crystals (like diopside and enstatite). Some basalts contain gemstones like corundum, zircon and garnets.

 Another volcanic rock is called kimberlite. Kimberlite pipes are the most major source of diamond.

a variety of obsidian specimens, including black obsidian, mahogany obsidian, snowflake obsidian, rainbow obsidian, and golden obsidian.
Types of Obsidian

Occasionally, varieties of volcanic glass, obsidian, are cut and fashioned as gemstones. Obsidian is an amorphous mineraloid with the hardness of approximately 5.5. Varieties of obsidian include:

Plutonic or intrusive rock

When molten rock solidifies within preexisting rock, it cools slowly, forming plutonic rocks with larger crystals. They tend to be coarse grained.

 Granite is a coarse grained intrusive rock which contains the minerals quartz and feldspar, and usually carries mica or hornblende. In some circumstances, granite undergoes "fractional crystallization", a process where slow cooling creates crystals of different minerals as they form at different temperatures.

 Minerals of the pegmatite group are among the last to be formed, often occuring as veins penetrating their surroundings.

Associated minerals that find their origin in igneous rocks:

Stages of the igneous or magmatic cycle

The stages of the igneous or magmatic cycle are as follows:

1-Stages of the igneous or magmatic cycle

  • Chromite
  • Magnetite
  • Titanium magnetite

2. Liquid magmatic phase (main crystallization) 1500-600 degrees C

  • Spinel
  • Zircon
  • Apatite
  • Peridot
  • Diamond

3. Pegmatite phase (rest crystallization) 700-400 degrees C

The residual part of the magma, which is rich in fluxes, is known as the pegmatite stage. The melt becomes a watery solution as solidification proceeds. Because of this fluidity, the liquids can penetrate fissures and cracks in the surrounding rocks. Under the concentrated pressure and temperatures, individual crystals form that can measure several centimeters, and occasionally several meters! The prismatic crystals grow perpendicular to the walls of the vein. Pegmatite veins are some of the best examples of gemstone formation.

  • Tourmaline
  • Beryl
  • Quartz
  • Feldspar
  • Zircon
  • Apatite
  • Brazilianite
  • Graphite
  • Muscovite
  • Lepidolite

4. Pneumatolytic phase 500-300 degrees C

Minerals formed in this phase form at lower temperatures and rising pressure. Superheated volatile components are involved. The most prominent of these components is water vapor, boron and fluorine gases. Under the influence of these vapors, other minerals are often formed in the contact zone of limestone.

  • Topaz
  • Euclase
  • Vesuvianite
  • Fluorite
  • Cassiterite
  • Sheelite
  • Wolframite

5. Hydrothermal phase 400-50 degrees C

This is a process associated with igneous activity that involves heated or super-heated water. Water at very high temperature and pressure is an exceedingly active substance, capable of breaking down silicates and dissolving many substances normally thought to be insoluble. This is the last stage of minerals that can be considered to be formed directly from magma.

Zircon to calculate the age of the earth

Zircon forms in granites deep in the earth's crust (plutonic rock). Through movement of tectonic plates, this granite is brought to the surface and starts mountain building. Through erosion, the granite (and the contained zircon) builds sediments which will eventually be buried deep enough to transform into metamorphic rocks.

Zircon has two important properties

  1. Relative high hardness
  2. Resistance to chemical attacks

Due to its hardness of 7.5 on the scale of Mohs, the zircons usually survive the sedimentary process intact. Because of its resistance to chemical attacks, zircon will survive the contact metamorphism process which is trying to attack it with heat and pressure. The latter is important as the liquid mass surrounding the zircon will cause a new rim to be formed around the old zircon, just like the formation of tree rings. This first cycle usually will take hundreds of million years. 

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