LoadErrors with specifically ruby 1.8.7 on OSX 10.7

This is on a clean install of OSX 10.7 Lion. Here's the actions I've taken:

I installed Xcode (4.1)

I installed RVM (1.8.0 ):

bash < <(curl -s https://rvm.beginrescueend.com/install/rvm)

$ echo '[[ -s "$HOME/.rvm/scripts/rvm" ]] && . "$HOME/.rvm/scripts/rvm" # Load RVM function' >> ~/.bash_profile

I installed ruby 1.8.7 and 1.9.2

rvm install 1.8.7
rvm install 1.9.2

I switch switch to 1.8.7

rvm use 1.8.7

I install a gem

gem install json

I use that gem in a ruby file.

require 'json'

I attempt to run that ruby file and get the following error:

main.rb.txt:1:in `re开发者_运维技巧quire': no such file to load -- json (LoadError)

I double check my gems:

gem list

..sure enough, it's installed.

*** LOCAL GEMS ***

json (1.5.4)
rack (1.3.2)
rake (0.9.2 ruby)
sinatra (1.2.6)
tilt (1.3.3)

This problem presents itself regardless of what gem I require. However, if I switch to 1.9.2, everything works fine.

I've uninstalled and reinstalled ruby 1.8.7 (along with every other version) with RVM. I've uninstalled and reinstalled gems with gem. What am I missing?

Additional information: Gem version: 1.8.6

Did you require rubygems first? You need to require that in order to activate the gem loader in your Ruby program:

require 'rubygems'
require 'json'