I'm a big believer in kitting out smartphones with a screen protector. A good-quality tempered glass screen protector is cheap insurance against scratches and chips, and can keep your smartphone looking new for longer, and can help improve the value if you want to resell your phone down the line.

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I always choose a good-quality tempered glass screen protector with a rated hardness of 9 Mohs.

Mohs is a scale introduced in 1812 by geologist Friedrich Mohs to compare the hardness of minerals. At position 1 is talc, a soft mineral, while at the top at 10 is diamond, the hardest. The unit is measured by scratching a material with picks of different hardness.

This makes Mohs a good way to measure how scratch-resistant a material is.

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When a good-quality tempered glass screen protector has a hardness of 9, it means that there are a limited number of materials that can scratch it. Gold, silver, and aluminum have a hardness of around 3, steel has a hardness of 4.5, teeth and volcanic glass have a hardness of 5, and hardened steel has a hardness of 8.

Note that sapphire, which is used as the crystal for some high-end wristwatches, including theApple Watch Ultra , has a hardness of 9.

But just how good are these screen protectors?

Let's find out.

Here I'm pitting theESR Armorite tempered glass screen protector against a few nasties.

View the ESR Armorite Screen Protector

$24.99 at Amazon

Note that I'm testing the screen protector on a soft, yielding surface. I believe that it would be even stronger on a flat, unyielding surface such as a screen, but I wanted to give the screen protector the toughest test possible.

Introducing the nasties.

Adrian Kingsley-Hughes/

I rubbed it with a rock...

The rock test.

Adrian Kingsley-Hughes/

... a key...

The key test.

Adrian Kingsley-Hughes/

... even a knife...

The knife test.

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... and the screen protector was completely unharmed.

Not a single scratch. Nothing at all.

I even pressed as hard I could into the screen protector with the knife.

Nothing.

Pressing the point of the knife into the screen protector.

Adrian Kingsley-Hughes/

Impressive.

Then I decided to up the ante by using a tungsten carbide glass breaker, which also has a hardness of 9.

I scratched and scratched and scratched, and while I could feel the point of the glass breaker occasionally catching on the tempered glass, after minutes of running it over the surface I hadn't made a perceptible scratch in the tempered glass.

Even a tungsten carbide glass breaker couldn't scratch the tempered glass.

Adrian Kingsley-Hughes/

So, about the only thing that's going to scratch your screen protector is diamond. So if you have some diamonds knocking around, keep them in a different pocket from your phone!

On a serious note, if you wear a diamond ring, be careful, because that can scratch things like your smartphone and car windows.

Also: How to apply a screen protector

The next question on my mind was, can atungsten carbide glass breaker break a screen protector?

Yes. It took quite a wallop, but it pierced it.

Tungsten carbide glass breaker breaking glass

Adrian Kingsley-Hughes/

So, tempered glass screen protectors really do offer a high degree of scratch protection, and even impact protection, but with the right tools you can still break tempered glass.

In my mind, a quality tempered glass screen protector is worth the money and can help prevent -but not completely stop -- you from damaging your smartphone's display.

Also: The best Apple Watch screen protectors

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