RVM gives you a separate gem directory for each and every Ruby version and gemset

This means that gems must be explicitly installed for each revision and gemset.

[~/.rvm]$ rvm list

rvm rubies

   jruby-1.7.3 [ x86_64 ]
   ruby-1.8.7-p352 [ i686 ]
   ruby-1.9.2-head [ x86_64 ]
   ruby-1.9.2-p290 [ x86_64 ]
   ruby-1.9.3-head [ x86_64 ]
=> ruby-1.9.3-p392 [ x86_64 ]
 * ruby-2.0.0-p0 [ x86_64 ]

# => - current
# =* - current && default
#  * - default

[~/.rvm]$ rvm gemset list

gemsets for ruby-1.9.3-p392 (found in /Users/casiano/.rvm/gems/ruby-1.9.3-p392)
=> (default)
   eleazar-pfc
   global
   rails3tutorial2ndEd

[~/.rvm]$ cat ~/.rvm/environments/ruby-1.9.3-p392\@eleazar-pfc
export PATH ; PATH="/Users/casiano/.rvm/gems/ruby-1.9.3-p392@eleazar-pfc/bin:
                    /Users/casiano/.rvm/gems/ruby-1.9.3-p392@global/bin:
                    /Users/casiano/.rvm/rubies/ruby-1.9.3-p392/bin:
                    /Users/casiano/.rvm/bin:$PATH"
export rvm_env_string ; rvm_env_string='ruby-1.9.3-p392@eleazar-pfc'
export rvm_path ; rvm_path='/Users/casiano/.rvm'
export rvm_ruby_string ; rvm_ruby_string='ruby-1.9.3-p392'
export rvm_gemset_name ; rvm_gemset_name='eleazar-pfc'
export RUBY_VERSION ; RUBY_VERSION='ruby-1.9.3-p392'
export GEM_HOME ; GEM_HOME='/Users/casiano/.rvm/gems/ruby-1.9.3-p392@eleazar-pfc'
export GEM_PATH ; GEM_PATH='/Users/casiano/.rvm/gems/ruby-1.9.3-p392@eleazar-pfc:/Users/casiano/.rvm/gems/ruby-1.9.3-p392@global'
export MY_RUBY_HOME ; MY_RUBY_HOME='/Users/casiano/.rvm/rubies/ruby-1.9.3-p392'
export IRBRC ; IRBRC='/Users/casiano/.rvm/rubies/ruby-1.9.3-p392/.irbrc'
unset MAGLEV_HOME
unset RBXOPT