9H, 10H or 12H Coatings
What do all these numbers mean and what should I look for in my next coating?

The most (mis)used term in the ceramic paint protection industry is 9H, 10H, or even 12H, used to market ceramic coatings.

These "ratings" are generally based on two different scales:
Mohs – a hardness scale for mineral stones (diamond, quartz, opal, etc.) – from 1 (talc) to 10 (diamond)
Pencil Hardness - film (ceramic coatings / PPF / paint) test method using the global standard ASTM D3363 - from 6B (softest) to over 9H (hardest)

Unfortunately, the two are often misused with all the 9H, 10H, or 12H marketing circulating in the industry.

And let's not even get into those who advertise 12H - neither scale goes that high nor will they ever show "independent" certificates for such ratings.

While these tests are generally carried out by independent institutes such as SGS or KTR, if you look more subjectively, you will see that the coating has been applied over a steel or glass test panel, which would already result in a high number, usually "Over 9H" for a pencil hardness test, which is the highest rating under ASTM and BS standards, so it can be quite difficult to know exactly what you're getting.

Coatings in general -
These coatings are very thin, under 5 microns, and in most cases, there is no way you can get a reading of the actual film hardness alone without also considering the hardness of the base substrate.
In the real world, these coatings are applied to much softer substrates like your car's paint, so while they will slightly increase the surface hardness, it should not be the deciding factor when choosing paint protection for your car, and a harder coating does not make it more durable or resistant.

Under real-world conditions, there are far more important factors than the hardness of the coating. Preparation before application is the most crucial factor, along with the coating's structural integrity and its ability to form a strong bond to the surface, its chemical resistance, hydrophobicity, and smoothness all play a role in making it a quality ceramic coating!

Can the highest rating still get scratched? Yes, of course! The reason is relatively simple. When the surface area of any material comes into contact with the coated surface, it is very small, but maintaining equal force, the pressure becomes so extremely high that the surface can then penetrate.
Much like how a mosquito can penetrate even the toughest surfaces like a rhinoceros's hide, which is hard as rock.

So if ceramic coatings are more about protection from dirt and grime and keeping the car clean for longer, is there a product out there that will stop scratches and stone chips?

Yes, Paint Protection Film (PPF) is a clear vinyl-like film that acts as an actual physical barrier on top of the paint, which absorbs impact and protects the paint from physical damage that would normally penetrate the paint and cause unsightly scratches and stone chips.