Folks, Gorilla Glass is a marketing ploy. It doesn't offer any real scratch resistance over normal glass, but the compound it's made of makes it less likely to shatter (two different things). When it comes to scratch resistance, you're looking at the Mohs scale.
Mohs scale of mineral hardness - Wikipedia, the free encyclopedia
You may be more familiar with this than you think. Diamond is a 10, the hardest substance known to man. Diamond can only be scratched by another diamond or a laser.
Glass has a rating of 6-7, depending on the hardness of the glass. Plastic is a 3-4.
If you take your keys over a glass screen (or a glass window), touch it to the surface, and rub with a little pressure (just enough to gain a degree of resistance), you WILL NOT scratch the glass. I've even scared the crap out of someone by hacking at my Zune HD's screen with a carbon-steel steak knife, and not a scratch.
So what will cause you to scratch your glass screen? Two things;
-a softer material (like metal) and force, not resistance. Yes, if you slam something into the screen, it may very well get a small crack, small enough that we see it as a scratch. Because, while glass is very scratch resistant, it easily shatters. It's force that gives us those small "scratches."
-dirt/sand - Sand (quartz sand, specifically) is an active ingredient in glass. If you get small particles of sand or other types of dirt on the screen and then wipe it off with your shirt (or even a microfiber cloth), you can develop small scratches in your screen. The most annoying is those who wipe with a circular motion, as it leaves rings in the screen. Want to avoid this? Take a SMALL amount of
Endust for Electronics and a paper towel and gently wipe off the screen. The liquid will act as a buffer between the screen and the dirt. Then, take a microfiber cloth (those little eyeglass cleaning cloths that come with screen protectors) and wipe off the residue left by the Endust and any pieces of the paper towel.
Do you need a screen protector?
Yes, I think you do. As scratch resistant as glass is, it can't account for acts of God. At some point, you will drop the device. At some point, a softer object will collide with it in a way that causes the screen to get a tiny crack that we view as a "scratch."
When you use a quality screen protector (SGP, Clarivu, Martin Fields), you're putting a plastic overlay over the phone. While this will scratch more easily than the glass screen, if applied correctly, it acts as a shock absorber, thus making it harder for the screen to shatter. It protects from scratches by taking the scratches itself, and it protects from shattering by absorbing the force of the blow.
The wet applications like the Invisible Shield are so thin that they don't noticeably increase shatter resistance. They'll just keep the screen together when shattered. The Invisible Shield is harder to scratch though, making it so that you don't have to replace it as often (it looks nicer longer than a normal plastic screen protector). Basically, it protects from scratches by deflecting them, but doesn't offer any real shock absorption.
