It may happen that at time while deleting the ruby gem the error can be :

ERROR: While executing gem ... (Gem::InstallError)
Unknown gem rake >= 0

This generally happens when there are multiple gem paths.

type ‘gem env’

RubyGems Environment:
– RUBYGEMS VERSION: 1.6.2
– RUBY VERSION: 1.9.2 (2011-07-09 patchlevel 290) [i386-darwin10.7.1]
– INSTALLATION DIRECTORY: /Users/hrastogi/.rvm/gems/ruby-1.9.2-p290
– RUBY EXECUTABLE: /Users/hrastogi/.rvm/rubies/ruby-1.9.2-p290/bin/ruby
– EXECUTABLE DIRECTORY: /Users/hrastogi/.rvm/gems/ruby-1.9.2-p290/bin
– RUBYGEMS PLATFORMS:
– ruby
– x86-darwin-10
– GEM PATHS:
– /Users/xxxx/.rvm/gems/ruby-1.9.2-p290
– /Users/xxxx/.rvm/gems/ruby-1.9.2-p290@global
– GEM CONFIGURATION:
– :update_sources => true
– :verbose => true
– :benchmark => false
– :backtrace => false
– :bulk_threshold => 1000
– REMOTE SOURCES:
– http://rubygems.org/

Gem is installed in multiple paths. Try deleting manually from one of the path

gem uninstall rake -i /Users/xxxx/.rvm/gems/ruby-1.9.2-p290@global