Installing RMagic 2.13.1 is generates issues on Mavericks when working with Brew... The problem is that the latest version of ImageMagick on Brew repos conflicts with RMagic version.. So you need to build older versions of ImagicMagick like 6.8.0-10 or 6.7.7-6....

Hence step one is to get the older version of ImageMagic..

cd $( brew --prefix )
git checkout 321b293 Library/Formula/imagemagick.rb 

This will get version 6.8.0-10. To install 6.7.7.6 checkout 7d951fb.

git checkout 7d951fb Library/Formula/imagemagick.rb

The next steps would be for 6.8.0-10.

brew install imagemagick

This will install imagemagick.. Now you need to link libraries since the library names have to generic without version information..

cd "`Magick-config --prefix`"
cd 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

Last step install RMagick...

gem install rmagick -v 2.13.1

Remember Brew is good tool for installation it is not necessary you use it. You can build ImageMagick from sources..