Try adding a require 'rubygems'
at the top of your source file. In 1.8 rubygems aren't required by default.
Thus said, regarding gem paths:
GEM_HOME
indicates where gems are installed. To find out what it is for your case do:
$ echo $GEM_HOME
This should display an RVM-related path, since RVM changes this environment variable in order to make the $ gem install
command install gems in its directories.