use gem which
.
$ gem which rack
/Users/sugyan/.rbenv/versions/1.9.3-p125/lib/ruby/gems/1.9.1/gems/rack-1.4.1/lib/rack.rb
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use gem which
.
$ gem which rack
/Users/sugyan/.rbenv/versions/1.9.3-p125/lib/ruby/gems/1.9.1/gems/rack-1.4.1/lib/rack.rb
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