If you've upgraded imagemagick to version 6.8.0-10 with homebrew recently, you may've noticed that rmagick gem won't work or install anymore.
Here's how to fix that:
$ cd /usr/local/Cellar/imagemagick/6.8.0-10/lib
$ ln -s libMagick++-Q16.7.dylib libMagick++.dylib
$ ln -s libMagickCore-Q16.7.dylib libMagickCore.dylib
$ ln -s libMagickWand-Q16.7.dylib libMagickWand.dylib
After that rmagick gem should install without any problems.
Update: Also, note that rmagick was last updated more than two years ago. So I recommend considering some other gem for working with images if you start a new project. Minimagick is quite good for example.