Believe it or not, use simple old toothpaste! NOT A GEL TOOTHPASTE, but the best is Baking Soda toothpaste "Arm & Hammer" if you can get it. Use a very soft close and dip it in the toothpaste and go in circular motions and then wipe clean with a soft dry cloth. You won't get it all out, but, it will take the worse away.
chrome
Yes, it is still a carbon steel with chrome and molybdenim molecules weaved into the micro structure.
Chrome is more durable and cheaper than stainless steel.
Use a magnet. If it sticks it's chrome. If not, it's polished aluminum.
Stainless steel
Pinballs basically come in two different materials; carbon-steel and chrome-steel. Carbon steel has less ability to retain magnetism. Chrome-steel can hold magnetism very easily (and quickly). Generally speaking, chrome-steel balls shine better than carbon-steel balls.
The fact that there are substances which are harder than steel.
No, for example high chrome stainless steel is not magnetic.
not sure.
Yes. A steel nail has a Mohs hardness greater than that of fluorite.
apatite with a steel knife feldspar with window glass
Use Flitz Stainless Steel & Chrome Cleaner With Degreaser.