From Mudrocks to Minerals - How do Gems Form?

The image above shows red Almandine Garnet crystals attached to a host rock in their natural environment. 

Would you believe that many garnets and other gemstones started as mud? As far-fetched as it sounds, it is indeed true!

Metamorphism is a process that is responsible for the formation of many semi-precious gemstones. Kyanite, garnet, and muscovite are a few examples of metamorphic minerals that can form from mudstones given the right conditions. 

In this article, you will learn about the basic rock classifications, how they are interrelated, and what the mineralogy of metamorphic rocks tells us about the condition millions of years ago.

General Rock Classifications

We have all learned about the 3 basic classifications of rocks in Geology 101.

• Igneous rocks are those that result when magma (underground) or lava (above ground) cools. The magma/lava composition paired with the amount of time that it takes to cool determines the resultant igneous rock type. Note that extrusive igneous rocks (from lava) are often referred to as volcanics. 
• Sedimentary rocks, also known as ‘clastics’, are comprised of an accumulation of cemented together rock fragments. These rock fragments are commonly referred to as ‘grains’ and can be of igneous, metamorphic, or sedimentary sources. At the highest level, grain size and mineral makeup determine the sedimentary rock type.
• Metamorphic rocks are those that have experienced events of elevated temperature and pressure. During the metamorphic process, minerals melt and then recrystallize into new minerals during the cooling phase.

The Rock Cycle

The rock cycle diagram helps us understand how different types of rocks are related, and how rock material is recycled.

Figure 1. Rock cycle chart

On the left side of the rock cycle chart, magma rises up to the surface where it cools and becomes an igneous rock. Being exposed to the surface environment, they undergo weathering, which breaks down the rock into smaller particles that are deposited at a lower elevation and eventually form into sedimentary rocks.

As sediments accumulate in a basin, the older ones become deeply buried. Over millions of years, thousands of feet of heavy and insulating sediments can accumulate above creating the high-temperature and high-pressure conditions necessary for metamorphism to occur.

Melting, Crystallization, and Metamorphism

The various minerals within a mudrock melt at different temperatures. As temperature increases, minerals melt according to Bowen's reaction series chart below.

Figure 2. Bowen's reaction series chart

Bowen's reaction series depicts the relative mineral melting points. As a rock heats up, at around 650ºC minerals begin to melt. First quartz melts, followed by muscovite, potassium feldspar, all the way up the chart to olivine at approximately 1400ºC. 

To put Bowen's reaction series in the context of semi-precious gemstones, the below list shows a few examples of popular gemstones that are classified under each category.

• Olivine: Peridot, Serpentine
• Pyroxene: Diopside, Bronzite, Hypersthene
• Amphibole: Prehnite, Nephrite
• Feldspar: Sunstone, Labradorite, Moonstone
• Muscovite Mica: Fuchsite, Muscovite
• Quartz: Amethyst, Chalcedony, Carnelian, Dumortierite

 The type of metamorphic rock that forms from a given a magma composition is dependent on the temperature and pressure that it was exposed to. The chart below highlights the types of metamorphic rocks that can form from mudrocks given different pressure and temperature conditions. 

Figure 3. Mudrock metamorphism chart showing the various metamorphic grades that result from increasing temperature and pressure.

High-grade metamorphism refers to rocks that were exposed to high temperatures and pressures, thus having a high degree of melting. And low-grade metamorphism simply refers to rocks that were exposed to lower temperatures and pressures, thus having fewer melted minerals.

As a rock and the melt cools, minerals crystalize in the reverse order from which they were melted. It's important to note that since the entire rock has not melted the elemental composition of the melt is different from the original parent magma. This means that during this cooling phase, different minerals will crystalize than those crystallized during the first cooling cycle of magma to igneous rock.

Metamorphic Zone Index Minerals

Mudrocks are exactly what they sound like, sedimentary rocks made-up of mud, sometimes also referred to as shale. As mudrocks increasing temperature and pressure, minerals melt as explained above, and when cooled, they form new minerals that can be the semi-precious gemstones that we have all grown to love.  

Figure 4. Metamorphic index mineral table.

 The table above shows how the presence of certain minerals can be telling as to the temperature and pressure conditions that the rock was exposed to. As shown in the chart, the presence of chlorite indicates low-grade metamorphism, while the presence of kyanite and sillimanite indicates high-grade metamorphism.

So next time you get out of your car and step in a puddle of mud, just remember, that millions of years from now that same mud might be transformed into a gorgeous gemstone, and you're just wearing its precursor on your shoe!

If you are interested to learn more about metamorphism, simply google search ‘types of metamorphism’ and you’ll be overwhelmed with pages of great information sources on the topic.

Links Relating to Metamorphic Garnet Deposits in Upstate NY.

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