I don't know what you mean by polished. Do mean something that shines like marble? I've had a high gloss porcelain on the floor of my bathroom for four years. It's not a mirror finish but it is certainly much shinier than eggshell.

There is not a scratch on my tile. It gets HARD use. It is not unusual for me to find a piece of gravel on the floor, which means it was stuck in the treads of someone's shoe from my gravel driveway outside. If someone can walk on it with gravel in their shoe and not leave a scratch, I don't know what it would take to scratch it!

I was very surprised to learn after the tile was installed, that the shine is not expected to last forever! I think the materials I was given say it will last 10-15 years. I don't know what I'm supposed to do about that when it happens!

Glazed porcelain is about as hard as glass.

While the glaze can be chipped(and the underlying porcelain cracked) it is that hard.