Don’t use the default ruby on Mac OS X.
Don’t run “sudo gem install gemname_here”.
Sooner or later, you will need multiple versions of ruby.
Sooner or later, you will need multiple versions of a gem.
Use chruby and ruby-install to manage different versions of ruby. Create a ~/.ruby-version to specify your default ruby version.
Use bundler to put your gems into your project folder instead of the global OS X folder.