brew install imagemagick

mdfind MagickWand.h

C_INCLUDE_PATH=/path/MagickWand.h gem install rmagick

Example:
C_INCLUDE_PATH=/usr/local/Cellar/imagemagick/6.7.7-6/include/ImageMagick/wand/MagickWand.h gem install rmagick

Might have to take these additional steps if you are using mountain lion.

http://robots.thoughtbot.com/post/27985816073/the-hitchhikers-guide-to-riding-a-mountain-lion

It appears it’s a problem reported on the Homebrew github repo (https://github.com/mxcl/homebrew/issues/16625) blaming rmagick itself not supporting newer versions of imagemagick. On that same issue (https://github.com/mxcl/homebrew/issues/16625#issuecomment-11519383), you can find this link: https://coderwall.com/p/wnomjg which actually worked for me. This is what he does:

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