Is Onyx a precious stone?
Onyx is translucent and gorgeous. Onyx can make an ordinary space look magical. But is Onyx a precious stone? Maybe it is. Maybe it is not. Knowing how different stones form in nature helps us understand their properties, their similarities, and differences and appreciate the forces of nature. Read on and decide for yourself if Onyx really is a precious stone.
The Origins of Rocks
If you missed my earlier post ‘Countertop Talks Part 2’ or just need a refresher on the origins of rocks, here it is.
On a very high level, there are three types of stones or rocks namely Igneous rocks, Sedimentary rocks, and Metamorphic rocks.
Igneous rocks are formed from cooled magma either on land or in underwater magma chambers under extreme conditions of heat and pressure.
Sedimentary rocks on the other hand form through the process of sedimentation which involves the deposition and compacting of sand, clay, and other minerals over time. This process can happen on land, meaning on beaches, deserts, or even on rivers and ocean floors.
Finally, we have Metamorphic rocks which as the name suggests form through the metamorphosis of stone! This process of metamorphosis takes millions of years in some cases and occurs under extreme environments of temperature and pressure.
If you would like to geek out and learn a little more about these different types of rocks, here is an article written by a Geologist: Geology of Natural Stone
What Type of Rock is Onyx?
Onyx is a sedimentary rock. There are two essential components that aid in the formation of onyx – limestone, and water. Yes, limestone is an essential precursor for onyx formation as well but, unlike marble, limestone does not transform into onyx.
Before we jump into the formation of onyx, let us quickly recap the limestone formation. Limestone is a sedimentary rock formed by the compression of shells, fossils, corals, plankton, and/or lime mud – all of which are composed of Calcite or Calcium Carbonate in shallow, warm waters. Given that limestone is formed in shallow waters, and under less strenuous conditions of heat and pressure, the minerals in the stone are not as strongly bound, making it porous. Check out part 4 of this series to learn more about Limestone.
Onyx Formation from Limestone
In some parts of the world, limestone formations get buried in the earth’s mantle. When groundwater encounters these limestone deposits, it dissolves the stone and carries the minerals AKA calcite or calcium carbonate with it to the surface. When this water eventually flows out onto the surface through springs, it brings along and deposits the calcite on the surface. When this process happens repeatedly, the calcite and other minerals in the water settle into the pores left by the previous deposits thus forming a stronger, non-porous chunk of calcite we call Onyx.
Wondering if this is not the same process of formation for travertine? It is! Check out part 5 of the series to see the similarities between travertine and onyx formation. The difference between these two stones is the temperature of the water, place of formation, and structure.
While travertine forms only near hot springs or geysers and in open pools, onyx can form from hot or cold water and can form in caves and closed areas as well. Travertine’s signature structure contains cavities which form due to the bacteria and microorganisms that thrive in the stagnant hot water pools and decay once the geyser or hot springs dries out while onyx has a smooth, flat surface that forms layer by layer with minerals being deposited into pores left by the previous deposits.
The major mineral in Onyx is Calcite just like in limestone and travertine. Calcite by nature is a clear mineral but can take on color from the small amounts of other minerals present in the water thus giving us a multitude of colors like the very popular Honey Onyx and the newer and brilliant Rainbow Onyx.
The Verdict on Onyx
Calcite is the major mineral in Onyx making it just as delicate and fragile as limestone and travertine. It can be etched when exposed to mild acids in the kitchen and easily scratched as well so the material must be sealed regularly. However, due to the formation process, Onyx does have a smooth and non-porous making it less susceptible to absorption.
Despite its drawbacks, Onyx still has a huge fan base. Onyx’s translucent quality makes it feel luxurious and gives it the ability to transform a space. Its limited reserves and translucent quality give it the status of a precious stone. It is commonly used as a bar top or as a feature wall in hotels and high-end homes.
Have more questions? Leave a comment below or hit me up! I love talking about stones and countertops. :)
Check out my other posts on the origins of different stones like part 2 on Granite, part 3 on Quartzite, Part 4 on Marble, part 5 on Travertine, and part 7 on Soapstone. Don't forget to subscribe to my Newsletter at the bottom of the page to be notified when new posts are published. You can also follow On my Lanai on Pinterest and Instagram to be notified of new posts.
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