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OS X Mountain Lion clean install Rails development environment setup using MySQL, imagemagick, homebrew and rvm. Rough steps, your mileage or your needs may vary:

- Install XCode from the App Store.
- Launch XCode > Preferences > Downloads > Install the Command Line Tools
- Install XQuartz version 2.7.2 http://xquartz.macosforge.org/trac/wiki/X112.7.2
- Install git, I used git-osx-installer version 1.7.11.3 http://code.google.com/p/git-osx-installer/downloads/list
- Install Ruby 1.9.3 with rvm, run this command on a terminal and follow the instructions: curl -L https://get.rvm.io | bash -s stable --ruby
- Install MySQL using the DMG installer, I used version 5.1.63 found here http://dev.mysql.com/downloads/mysql/5.1.html I use this to get the files needed for the mysql2 gem to install, I don't install the prefpane or the init script since I use MAMP as my MySQL server on my Mac.
- Install homebrew: https://github.com/mxcl/homebrew/wiki/installation 
- Install ghostscript, from a terminal: brew install ghostscript
- Install imagemagick, from a terminal: brew install imagemagick

This was enough for me, I ran bundle from a project I'm working on and all the gems downloaded properly including mysql2 and rmagick.