I had a lot of trouble installing ruby 2.0.0 on mountain lion.  Tried various things recommended on different forums and blogs, but the one that was easiest and worked best for me was the one written by Christopher Kaukis on the Interworks blog.  In short, the steps are as follows.

1.  Install the latest version of Xcode from the App Store.  (at this time of writing it is version 4.6)

2.  Go to xcode’s preferences and install the Command Line Tools

3.  Install Homebrew.

4.  Check requirements for RVM and install it.  (I use the multi-user install)

  • brew tap homebrew/dupes
  • brew tap –repair homebrew/dupes
  • brew install autoconf automake libtool pkg-config apple-gcc42 libyaml readline libxml2 libxslt libksba openssl sqlite
  • curl -L https://get.rvm.io | sudo bash -s stable

5.  Upgrade RVM to the latest repository source version and install ruby 2.0.0 then start using it.

  • rvm get head
  • rvm install 2.0.0
  • rvm use 2.0.0