Gemstone Formation

For millions of years gemstones have formed beneath the surface of the Earth in a variety of different environments.

Traditionally, gemstones fall into three rock classifications: Igneous (Magmatic), Metamorphic and Sedimentary. Igneous or Magmatic rocks crystallize from molten magma, lava or gases. Sedimentary rocks crystallize from hydrous solutions on or near the Earth’s surface, while Metamorphic rocks re-crystallize from existing minerals that have been subjected to great pressure and high temperatures.

Gemstone formation is generally classified into four processes:

1) Molten rock & associated fluids
2) Environmental changes
3) Surface water and
4) Formation in the earth’s mantle.

While potentially confusing, it should noted that some gemstone varieties are formed by more than one process.

Molten Rock & Associated Fluids

Molten rock & associated fluids are minerals that are formed in the magma or its escaping fluids. They are created by heat, deep within the earth. Molten rock & associated fluids are further classified into magma crystallization, gas crystallization, hydrothermal and pegmatites.

Magma Crystallization

As magma cools its various elements combine to form minerals. When one mineral forms, the available ingredients, temperature and pressure gradually change to create different minerals. While one mineral will occasionally crystallize nicely, if the conditions are not suitable, no crystals will form and the magma will simply cool into aggregate rocks (solid masses of small, interlocking crystals).

Before all the magma can crystallize it will break into the crust and rush towards the earth’s surface. When the pressure and temperature are too low for crystallization, the rest of the magma cools into fine-grained rocks with the original crystals distributed in “phenocrysts” throughout the rocks interior. Gems formed in these conditions include Sapphire, Ruby, Moonstone, Garnet and Zircon.

Gas Crystallization

While some gems grow on a solid base, others form inside gas bubbles. Gas bubbles are formed during a volcanic eruption when rising magma undergoes a rapid reduction in pressure. These bubbles often contain high concentrations of certain elements and with the right combination of temperature and pressure, gems including Garnet, Topaz and Spinel are formed.

Hydrothermal

Hyrdothermal liquids are created when water and heat interact with magma deep inside the earth. These liquids contain water, carbon dioxide, special elements (such as fluorine and beryllium) and volatiles (substances that are readily vaporized) that have escaped from the magma through fractures and fissures. Hydrothermal liquids may dissolve minerals or combine with ground water as they solidify and form mineral veins. If combined with the right temperature, pressure, time and physical space, gems including Amethyst, Topaz and Emerald are formed.

Pegmatites

When magma in the upper part of the mantle becomes concentrated with volatiles it cools into a cavity called a pegmatite. As the molten rock begins to solidify, the elements begin to crystallize into gems including Topaz, Tourmaline, Kunzite, Aquamarine and Morganite.

Environmental Changes

Environmental changes, such as changes in temperature or pressure, can alter existing minerals into something new. This process is called metamorphism and it is divided into two types, contact metamorphism and regional metamorphism.

Contact Metamorphism

Contact metamorphism occurs when magma forces its way into an existing rock. The intense heat melts these rocks and re-crystallizes new minerals that are stable at higher temperatures. Gemstones formed by contact metamorphism include Garnet, Diopside, Spinel and Lapis Lazuli.

Regional Metamorphism

The earth is composed of continental plates that float on the mantle. As some of them compete for the same space, their interaction is responsible for the formation of geographic features such as mountains. The intense heat and pressure generated by these geological events can cause minerals to become unstable, changing them into new varieties over time. Polymorphs are gemstones that re-crystallize into a new crystal system during regional metamorphism. Examples include Andalusite, Kyanite, Sillimanite, Tanzanite and some varieties of Garnet. In contrast, pseudomorphs like Tigers Eye change their chemistry through atom-by-atom replacement during regional metamorphism.

Surface Water

Rain plays an important role in recycling minerals and creating new gems. As water passes through the earth, it picks up various chemicals that can react with each other in a variety of ways. When a dry season occurs after a period of heavy rainfall, water tables fall, leaving behind deposits of different minerals in seams and cavities. Depending on what chemicals the water has reacted with, gemstones including Opal, Turquoise, Malachite, Amethyst, Agate and Azurite are created.

Gems Formed in the Earth's Mantle

The earth’s mantle is composed of molten rock and gases called magma. It is 83% of the earth’s volume and 3,000 kilometers thick. Near the center, the mantle is extremely hot and kept in constant motion due to currents of heat. Where the mantle and crust meet, a tumultuous zone of high pressure and temperature is created.

Peridot and Diamond are examples of gemstones that crystallize at extremely high temperatures. Peridot deposits in Arizona were created on rocks floating in the mantle, approximately 32 to 88 kilometers below the earth’s surface. Diamonds crystallize in the magma 176 to 240 kilometers below the earth’s surface where the temperatures are higher and the magma is very fluid.