I wish I could speak intelligently to the differences of composition of the tiles versus porcelain. I agree, they are amazing. To look at and hold a tile, you think "This is gonna handle hitting the atmosphere at Mach 18 or whatever and not disintegrate?", but they have and they do.
The most critical tiles are on the underbody of the shuttle. Those are not very likely to be either damaged or suspect, tho' they are replaced after missions frequently in small numbers and in a routine manner. But to look at them , "This is gonna get me home?".
Space Flight is the ultimate act of faith... but you better believe in your vehicle.
I read specs on an FR thread that said the tiles would protect to 2300F. Then I watch the Nasa press conference that said reenty was over 3000F. The 2300 stuck with me because that's the temperature at which porcelain "cures." You support porcelain in a kiln with this silica stuff that looks like quilt batting...the silica stuff looks exactly the same after firing as before you fire the kiln. It's called "prop"...I wonder if the tiles aren't made largely from this stuff.
Anyway, just a throwaway, an idle curiosity. Did you know that there are ceramic knives far harder and sharper than the best steel?