9H: the industry's most repeated number, and the one that distinguishes least

The confusion is understandable. The Mohs scale taught in school runs from talc (1) to diamond (10). If 9H were a Mohs figure, a screen protector would sit almost at sapphire. It does not.

The 9H comes from a pencil hardness test, an industrial method for coatings. Leads of increasing hardness are drawn across the surface and the first one to leave a mark is recorded. The pencil scale runs from 9B (softest lead) to 9H (hardest). "9H" therefore means the protector withstands the hardest pencil in the set — nothing more.

Translated into Mohs, the tempered glass in these products sits at around 6. Far less impressive, and considerably more informative.

That 6 on Mohs explains what anyone who has taken a phone to the beach already knows. Quartz — the main constituent of sand and of a good share of household dust — sits at 7 on the Mohs scale. Harder scratches softer, so sand scratches the protector no matter what 9H figure is printed on the box.

The inverse is better news: keys and coins are alloys around 4 to 4.5. They do not scratch glass, and never did. The pocket-full-of-keys worry is a holdover from the era of plastic screens.

So a protector does not prevent the sand scratch — it takes the scratch instead of your screen. When it is marked, you replace a cheap component rather than living with a scarred display.

• Thickness: 0.25 to 0.33 mm is the usual range. Thicker absorbs impact better; thinner is less noticeable under the finger and is the better choice if your phone has an ultrasonic fingerprint reader.
• Full-surface adhesive rather than edge-only glue. Full adhesive prevents the rainbow effect, bubbles and fingerprint-sensor failures.
• Oleophobic coating: it is what makes the finger glide and lets fingerprints wipe away. It is also the first thing to wear off, and the real difference between a known brand and an unbranded one after three months.
• 2.5D edges: the rounded edge stops the rim lifting and stops it chipping on a side impact.
• Installation frame: worth more than any hardness figure, because the most common failure is not the material but a crooked or dusty fitting.

If a listing boasts only about 9H and mentions none of these, it is describing exactly what its competitors claim too, and staying quiet about what separates them.

Prices change daily, so the price and availability that count are always the ones shown on Amazon once you follow the link.