I have seen other people with the same issue of installing RMagick on Mountain Lion However none of the suggested solutions have allowed me to successfully install rmagick.
I know this is a pretty old issue but it did happen to me recently. I am posting this for those that have tried all other answers and weren't able to resolve so far. Bumping the rmagick gem to version 2.13.4 has worked with Ruby 1.9.3 on MacOS.