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Comparing Natural Travertine vs Onyx

As you may have discovered in your research of natural stone materials, there are many from which to choose. Two of these we will look at in this article are travertine and onyx. We will take the approach of comparing travertine vs onyx to peer at the similarities between the two. Then we will see what makes them different from one another. But first, let's see why we are comparing these materials in the first place.

Why Compare Natural Stone?

The information regarding natural stone can be somewhat challenging to sift through. And even while researching materials you will likely find that some information conflicts with other data you have found. One of the main reasons for this is that the terminology used varies from one source to the next. For example, if you talk to a geologist, you will get one set of definitions and labels. Yet, if you discuss the same topic with someone in the commercial stone industry you will get different terms and descriptions. This can get confusing quickly.

Comparisons are practical for at least a couple of reasons. First, when you compare two things, it guides the focus to how they are the same and how they are different. This allows for distinctions to be made. Distinguishing one thing from another is an effective way of learning about a topic. In our case it also allows for explanations to be made regarding the different terminology used.

Approaching Terminology

Let's briefly mention why scientists and commercial professionals use different terms and definitions when naming and describing stone materials. To boil the reason down to simple terms, we could says that commercial stone professionals use terminology to communicate practical aspects of the material. Conversely, geologists use the technical aspects that make a material what it is. After all, clearly defined, detailed definitions is a large part of science. In our discussion of travertine vs. onyx we will not stick to one approach or the other. Rather, we will make an effort to explain the comparison in away that is easily understood. Without further ado, let's get into the material.

How Travertine and Onyx Are Similar

We are going to start our discussion with the similarities between travertine and onyx for a very good reason. And that reason is because these two materials are different versions of the same material. That's right, these materials are extremely similar to one another and yet, geologically speaking they are still different stones. And this might be a good place to mention a difference in terminology. In the commercial stone industry, the term "onyx" describes a rock that geologists refer to as "banded calcite". Whereas scientist have labeled a specific mineral with the term onyx. The chemical formula for onyx is SiO2 (or silica). But banded calcite's chemical formula is actually CaCO3 (or calcite). Now, you might be thinking, "those are two completely different materials." And you would be right. The takeaway here is this: both travertine and "onyx" (the version used by the commercial stone industry) are made of calcium carbonate (calcite). Thus, they are both calcareous materials.

What is Calcareous Stone?

Calcareous stone is natural stone that that is primarily made up calcium carbonate. There are a few of these and you have most likely heard of each of them at one time or another. Calcareous stones include the following:

  • Marble
  • Limestone
  • Travertine
  • Alabaster
  • Onyx (commercial term)

Both of the materials we are discussing in this article are in that list. The fact that both onyx and travertine are calcareous means they have some very similar traits. Let's see how the calcite content affects these two stones.

How Calcite Content Affects Onyx and Travetine

Both tavertine and onyx are composed primarily of calcium carbonate and this has a very specific effect on these materials. The minerals that are in a material will ultimately have an impact on the characteristics of that material. And as we will now discuss, the fact that both of our compare stones are calcareous means they share some characteristics. What are these?

Calcium Carbonate is Relatively Soft

The first trait that these materials hold in common is that they are both relatively "soft" as far as stone is concerned. Natural stone hardness is measured and expressed using a mineral hardness scale. The scale ranges form 1 to 10. The higher the number, the harder the material. Calcite is on the low end of that scale. As a result, both travertine and onyx come in at 4-5 in hardness. So when we say that these are "soft" we simply mean that they are not as hard as other natural stone types, such as granite or quartzite. Travertine and onyx are still types of rock, it's just that there are harder rocks out there.

The hardness of a material will affect its resistance to scratching. The harder the material the more difficult it will be to scratch. In fact, scratching a stone is precisely the manner in which its hardness is measured. Scratch tests are done by using an object with a known hardness and trying to scratch the material under test with that object. The bottom line though is that both travertine and onyx are softer that other natural stone materials.

Calcite Reacts With Acid

Another common property that exists between travertine and onyx is the fact that both of these materials react with acidic substances. Calcium carbonate neutralizes acid. Thus, calcareous stone is affected by the presence of acidic liquids. When this happens the result is an etch that appears as either a dull or dark spot on the stone. If a stone manages to get etched, it may be treated with an etch remover for calcareous stone and then sealed using a stone sealer that is food safe. Additionally, treating with a spray polish for stone will make the area look as close to its original gloss after you have treated the etch.

Differences Between Travertine and Onyx

Up to this point we have discussed travertine vs. onyx (banded calcite, geologically speaking) in the context of how they are similar to one another. Yet, these materials are two distinct kinds of rock. Let's look at how these materials differ.

Forming in Wet Environments

Although travertine and onyx form in wet environments, there is a distinction between the environments in which these materials form. Travertine is commonly found forming in the presence of hot water, think at the mouth of hot springs in a precipitous manner. And although onyx can form where there is hot water, it also forms where the water is cooler. The environments and the manner in which these stones form results in a very different and interesting visual appearances. Let's look at those now.

Travertines Interesting Porosity

Perhaps the most recognizable feature of most travertine is that it is peppered with holes. These voids, or pores, are the hallmark of this natural material. And even though the holes in the stone are usually filled when the stone is processed, the filler that is used to strengthen the stone does not remove the rustic appearance of the stone. Rather, it enhances it. Fillers that are a complimentary color are used as well as transparent fillers that keep the original look while still filling the voids in the stone. Once you see travertine, it is hard to miss it after that.

The Translucency of Onyx

On the other hand, natural onyx also has a distinctive trait that sets it apart from many other materials. Onyx (banded calcite), has translucent bands of color that run through the stone and alternate with the calcite. These translucent band of color are usually a brown or honey colored hue but there are other colors too. The interesting thing about Onyx is that it can be backlit and the light will shine through the translucent areas of the stone much more easily than it does the rest of the material. This makes for an astounding presentation and is very identifiable.

So there you have it, two similar materials. Each with its own specific, distinct, look. Even though natural travertine and banded calcite (what we have been referring to as onyx) are composed of the same primary mineral and are similar in many ways, they are different. The difference in this case can be seen in their notable visual characteristics. Both of these stones however are very desirable due to their interesting and distinctive looks.