Hardness for tempered glass is like strength for steel, clarity for glass, flexibility for plastics, or water resistance for rubber — it’s the biggest selling point for a tempered glass screen protector. That’s why a 9H callout is generally placed somewhere prominent on a package. It refers to Mohs hardness scale of minerals in a range from 1 to 10. On this scale, regular glass is placed at 5 and a diamond is at the highest 10. This hardness is measured by the ability of minerals to scratch each other’s surfaces. Glass can scratch anything below a 5, such as iron, but glass can be scratched by anything above a 5, such as quartz.
Tempered glass didn’t exist in Mohs’ time (early 19th century), so it didn’t make it to the chart back then — but that was changed as soon as the tempered glass screen protector entered on stage. With a little marketing push, it was promoted to higher ranks, bypassing quartz and topaz, and positioning itself amongst the hardest minerals out there. How can anyone argue with 9H, considering most people don’t carry diamonds in their pockets? Sounds great in theory, but there is a catch.
9H Tempered glass does not exist.
In the world of screen protectors, the ability to protect from scratches is a pretty big deal. Keys in your pocket, the sand on a beach — those micro-scratches can weaken the screen over time, making it more susceptible to breaking. Despite the fact that our screen protectors are made from plastics, not minerals (and can’t possibly exist on Mohs scale), we test our screen protectors with a number of tools including a Deluxe Hardness Pick Set from Mineralab.
We’ve lost count on how many tempered glass screen protectors came through our doors — we have yet to see one that bypasses 6 or 7 (We welcome anyone to prove us wrong).
But there is a bigger problem that is worth mentioning.
Mohs hardness is not a suitable measurement for screen protectors.
Mohs Hardness is a rough measure that is determined by observing whether the material is scratched by another material of defined hardness. For that reason, Mohs test, while greatly facilitating the identification of minerals in the field, is not suitable for accurately gauging the hardness of materials.
Corning, the maker of the famous toughened Gorilla Glass, stays away from defining an accurate Mohs hardness number. To the consumer, it might look like a sign of weakness (after all, it was calculated that the Mohs hardness number for Gorilla Glass is somewhere around 7), but remember that Corning isn’t marketing their products directly to consumers — they market it to industry specialists who pay more attention to other factors, aside from the observational scratch tests.
9H might be the greatest marketing gimmick in the history of mobile accessories, but hardly a measurement to be taken seriously if you are shopping for a good screen protector.
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