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So, rocks are hard. But the scale we use to rank them, the Mohs scale, is only really good at quantifying that for one kind of hardness, and topaz is a perfect stone to talk about to explain that. And you can check it out in our SciShow Rocks Box subscription!

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Mohs Hardness Scale (U.S. National Park Service).
Topaz | Geoscience Australia
Cleavage of Minerals | Geology In
The Largest Faceted Gemstones in the World | Geology In
Moh's hardness scale compared to indentation hardness

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Rocks are hard.

Brilliant observation, I know. But if you’ve ever crumbled some  mica or broken open a geode, you might have noticed that some  rocks are harder than others.

And as it turns out, scientists  have got a measurement for that! The Mohs hardness scale  ranks the hardness of stones. The higher the number, the harder the stone, all the way up to ten, which is diamonds.

So a stone that’s an eight on the scale,  like topaz, must be pretty tough, right? Well, it turns out there’s some  underlying nuance to the Mohs scale, and topaz just happens to be the perfect mineral to highlight as we unpack all of that. So grab your pickaxes and let’s dig in! [♪ INTRO] So you may have heard of this thing  called the Mohs hardness scale, which is used to rank minerals.

And on the Mohs scale, every mineral  falls somewhere between 1 and 10. Minerals ranked lower on the scale can be scratched by ones higher up on the scale. And some common non-mineral objects  get scores too, like fingernails and nail nails, which is  helpful when you’re trying to identify minerals out in the field.

No need to carry around an array  of minerals and have a scratch-off. So for example, fingernails have  a Mohs hardness of about 2.5, which means you can scratch any mineral  ranked below 2.5 with your fingernail. For example, Talc and gypsum  are both softer than 2.5.

Which means that any geologist  can pick up a mystery rock and get a sense of what  kind of mineral it might be, based on whether they can  nick it with their fingernail. It’s not really working on this one. Now at the very top of the Mohs  scale is diamond, with a score of 10.

Diamonds can’t be scratched by  any naturally occurring mineral, except for other diamonds. At number 9, we have corundum,  a hard mineral that’s better known as ruby when  red, or sapphire, when blue. But for more on sapphires, check  out our very first Rocks Box video!

And right at number 8 on the scale, we have topaz. There are other minerals at number 8  too, but most versions of the scale have topaz as the reference mineral for 8. She’s like, the 8 to be.

But here’s the thing about that. Topaz is actually easier to break  than a lot of the other eights, and it’s even easier to break than some  things that are way lower on the scale. So what’s up with that?

Well, let’s start  with a bit of info on topaz in general. Topaz is a silicate mineral kinda like quartz, but with aluminum and fluorine in the mix too. And topaz’s hardness comes  from the strong chemical bonds between those atoms, which makes  it resistant to being scratched.

Pure topaz is completely colorless,  but when trace impurities get added into the mix, the resulting  crystal can be any number of colors. The most common ones are  pale blue and golden yellow, while the rarest topaz colors are red and pink. And these crystals can be truly massive.

The largest faceted gemstone in  the world is actually a topaz. It’s called the El Dorado Topaz, and the  raw crystal was discovered in Brazil. This stone weighs in at a massive  6.2 kilograms, or 31,000 carats!

But while topaz is a pretty gemstone, it’s not as tough as the other  eights on the Mohs hardness scale. For instance, there’s a stone called  beryl that typically ranks about 7.5 to 8, which means it’s supposed to be softer than topaz. But beryl tends to be a lot  more durable than topaz.

And the reason for that is that  hardness isn’t the only property a stone can have related to its durability. There’s also cleavage, and topaz  happens to have perfect cleavage. But get your mind out of the gutter,  alright?

Cleavage is a real geology term. Cleavage is a property related to how crystals  break, or cleave, along certain lines. Minerals with strong cleavage break along  predictable and well-defined planes.

And there are different types of cleavage, and they all vary depending on the  internal structure of the crystal. Basically, the way atoms are  arranged defines fracture lines, and when a force strikes a crystal, the  crystal tends to break along those lines. It’s kind of like those chocolate oranges  that you need to smack to break apart.

The slices are pretty hard, but the  entire structure has weak points. And when you strike those weak  points, the whole thing falls apart. The atoms in topaz are strongly  adhered in certain directions, but only weakly attached in  others, leaving weak points.

So all you need to do is smack it  at the right angle, and it’ll crack. There are a few different kinds of cleavage, too, all based on where the weak points  are arranged inside that crystal. And some crystals can even have more than one plane of cleavage in the same crystal.

For instance, having two planes  is called prismatic cleavage, and three is called cubic cleavage. But there are a lot of others, and we  don’t have time to go into them all. But because topaz has perfect  cleavage, it breaks easily along a bunch of different planes,  creating nice, defined lines, with both sides having an  almost mirror-like sheen.

But the Mohs scale doesn’t  take cleavage into account, because it just characterizes  one type of crystal hardness, which is scratch resistance. But scratch hardness is different  from other types of hardness. For example, in lab settings,  researchers also often want to quantify a mineral’s indentation hardness,  or its resistance to deformation.

In those tests, instead of scratching, they look at how well a  mineral holds up to pressure. A sharp tip gets pressed into  the surface of the material, and researchers measure how much the tip digs in. Indentation hardness and Mohs hardness  usually go together, but not always.

In fact, under indentation hardness testing, topaz barely holds up better than  quartz, which has a Mohs hardness of 7. And while topaz will scratch a steel file, the steel holds up to indentation  testing way better than topaz does. So when you’re evaluating a  mineral, it’s important to consider all of its material properties, and keep  the Mohs scale in the right context.

That shouldn’t be too hard, should it? And hey, if you wanna give this a try yourself, you can sign up for a SciShow  Rocks Box subscription and receive a topaz to scratch,  squeeze, or just to admire. Every month, our Rocks Box subscribers  get a sample of a mineral or fossil that we hand-picked, just for you.

They’re always ethically sourced, so there’s no guilt involved in expanding your collection. But the spots do sell out fast,  so head over to Scishow. Rocks now if you’re interested in that subscription.

And thanks for watching! [♪ OUTRO]