"I was thinking over the idea for right coin detection. can we detect the metal type by putting the metal between two windings"

Yes, that is the principle used by the coin discriminator in a vending machine's coin mechanism.
However, coin mech manufacturers have spent many, many, many man-years of development effort honing, testing, & tuning their algorithms to give reliable, accurate discrimintation - after all, we're talking money here!
The discriminators use several coils, and some very clever driving, detection, and lots of DSP.

There are also lots of other considerations - like the mechanics of control the coin's speed through the "gate", jam prevention, preventing people dangling a coin on a string, etc, etc, etc

Discriminators come in various "grades" depending on the accuracy required (ie, the values involved!)

http://www.coinco.com/coin/
http://www.nri.de/eng/index-eng.htm
http://www.jofemar.com/indexing.htm
http://www.meiglobal.com/