The Hardest Rock and Hardest Mineral on Earth
The Hardest rock on earth are metamorphic type rocks and the hardest mineral on earth is diamond. Metamorphic rocks are known to be the hardest as they are rocks with a high concentration of minerals that have formed through heat and pressure as a more solid dense rock. Diamond is the hardest mineral as it is the most difficult to break or cut.
When measuring the hardness of rocks, many like to use the Mohs scale of harness used for minerals. Basically the Mohs scale of hardness measures the ability of one mineral to visibly scratch another mineral. It is said that using the Mohs scale of hardness on rocks can be misleading as rocks will vary with their mineral contents.
This is understandable and it makes it difficult to pin down which rock is actually the hardest rock. Based on my research on the concept of “hardness” it would appear that “in general” metamorphic rocks are thought to be the hardest. Their mineral concentration can be greater as you couple that with the pressure and heat as well.
Metamorphic Rocks, the Hardest Rocks
Metamorphic rocks are thought to be the hardest rocks you will find on earth. The hardest metamorphic rocks will be the ones that undergo large amounts of heat and pressure deep within the earth without melting. These types of metamorphic rocks will typically be more harder than other rocks.
Igneous rocks, like granite, are very hard as well, but can fall slightly short of some of the hardest metamorphic rocks. Igneous rocks are rocks that have melted to magma and then cooled. Sedimentary rocks, the third type of rock, consists of bonded sediment. Sedimentary rocks typically don’t have the heat and pressure in their bonding to be as hard as igneous or metamorphic rock.
It is important to note that when it comes to rocks, hardness of a certain type of rock can vary. There will be plenty of metamorphic rocks that can fall short in hardness of say granite.
Quartzite is commonly considered as one of the hardest metamorphic rocks. This metamorphic rock is typically harder than the igneous rock granite. Quartzite is very hard due to the large amount of the mineral quartz contained within it.
Did you know that topaz rich granite can be harder than some quartzite. The mineral topaz has an 8 on the Mohs scale of hardness. With a high concentration of this mineral, it can make granite quite a hard rock.
Using Compressive Strength as a Measurement
So if using the Mohs Scale of harness is not always the best measurement of hardness, how about using compressive strength?
Compressive strengthen is the max amount of compressed stress put on something, say a rock, before it deforms by a specific percentage, say 5% or 10%.
This type of measurement does not appear similar at all with the Mohs scale concept of hardness. So is it a good measurement to use for hardness?
The divide here seems to be between the concept of surface hardness widely used in mineralogy, compared to the actual “toughness” or “strength” of the rock under pressure. I’m not going to lie here, I do like both measurements but I am still not sure what makes the “Hardest/Toughest/Strongest” rocks.
For fun, I decided to do a little research to find out which rocks have high compressive strength. My findings were very interesting.
Flint, Chert and Novaculite, according to one site have the highest compressive strength sitting at around 450.00 N/mm2. I would not have expected that. Even though flint for example is high in quartz content, it is a very brittle sedimentary cryptocrystalline form of quartz. All three have a hardness of 7 on the Mohs scale of hardness.
Despite having a 7 on the Mohs scale, being brittle makes it chip and break easier than other rocks.
Quartzite on the other hand sits at 115 N/mm2 and Granite at 175 N/mm2 according to this same site. This would appear to make these two rocks weaker when under pressure than that of flint.
Some additional research found a different measurement, “MPa”(a unit of pressure or tension), on these rocks. Flint has an estimated 600 MPa, while quartzite has an estimated up to 300 MPa.
I am finding both measurements interesting but it does appear to confuse the topic of hardness in rocks. Perhaps that is why many have just gone the route of measuring rocks as they do minerals, using the Mohs scale of hardness. Perhaps down the road someone will come up with a good measurement combining the two or doing something completely different. (Maybe they already have and I have not found it yet.)
The Hardest Mineral, Diamond
Diamond is the hardest mineral known on earth. It is used on the Mohs scale of hardness as the definition for 10, being the highest and hardest on the scale. Diamond is hard enough to use as a cutting tool for just about everything.
The diamond is a pure form of carbon type mineral. Only a diamond can cut another diamond. Many around the world use diamond as a means to cut hard materials as well as use diamonds to adorn our bodies with. If you have the money that is.
Diamonds that are used for cutting purposes are not quite the same as the diamonds people use as jewelry. Cutting diamonds are of poor visual quality and they are actually quite abundant on our earth. The quality diamonds are the ones that make the grade for wearing.
There is some controversy around the world that also suggests that quality diamonds are not very rare either. It is said diamond selling companies will hoard as many of these quality diamonds to help control the prices of the mineral.
Will Diamonds Remain the Hardest Known Material?
Scientists have already discovered new materials harder than a diamond, or technically more durable. These two materials are called wurtzite boron nitride and the mineral lonsdaleite. Both are similar to a diamond but atomically structured to be more durable than a diamond.
Wurtzite boron nitride and lonsdaleite have been found in only small amounts so far on our earth. Additionally, only small amounts of these materials have been man-made. It is said that lonsdaleite can be formed from a meteorite containing graphite in it, just to give you an example of natural rarity on our planet.
Wurtzite boron nitride has a similar structure to a diamond, however it is comprised of some different atoms. Lonsdaleite is essentially a diamond but its carbon atoms are patterned in a more durable shape. Wurtzite boron nitride is said to withstand roughly 18% more stress than a diamond, and lonsdaleite withstands roughly 58% more stress than a diamond.
Other Hard Minerals
With diamond being the hardest mineral on the Mohs scale, other hard minerals such as corundum and topaz take up positions 9 and 8 on the same scale. Corundum and topaz are popularly known as very hard minerals, just after a diamond.
Many like to use a masonry drill as a comparison for corundum and topaz. A masonry drill has a hardness on the scale at around 8.5, just in-between the two minerals.
Topaz is a well-known mineral and is often used in jewelry. This mineral is often used as a more affordable replacement to the wearing of diamonds. Topaz will carry a very nice look in similar to that of diamond.
Different Uses of Hard Rocks and Hard Minerals
In my research with hard rocks and hard minerals, I have noted that the uses can vary greatly with rocks and minerals that are harder.
It seems the softer rocks and softer minerals tend to have more practical everyday uses, even in the consumption of minerals. Harder rocks and minerals seem to have more use in applications such as jewelry, display, and construction. Minerals seem to be more so in this particular observation, as harder minerals make great gemstones.
I have written several articles on various rocks and minerals in my “Rock and Mineral List”. I encourage you to go take a look there as there are several great examples of rocks and minerals describing many of their properties.
With all this information on the hardness of rocks and minerals, you might be interested in how they all come to be. Have you heard of the rock cycle? Maybe you have but you don’t know much about it? The rock cycle details the changes each of the 3 types of rocks go through on our earth, see my article “Diagram of the Rock Cycle”.
Conclusion
It is pretty easy to note that a diamond will be the hardest mineral. As for the hardest rock, it does seem a bit suspect on how to define that. In this article I have generalized that metamorphic rocks will be the hardest based on the geological way it was created with some other online research.
The question I am left with is, how do we properly measure the hardness of a rock? The Mohs scale does not seem quite right, but falls in line with the concept of hardness in minerals. Compressive strength does not seem to fit properly unless maybe you are lookin for strength under pressure, not hardness.
