WinstonYW

Specific to Mac OS X 10.7.*, ImageMagick v6.8.0-10 and RMagick v2.*.

The correct way to do it:

$ brew remove imagemagick
$ brew install imagemagick --disable-openmp --build-from-source

$ 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

The errors I got when I didn't install ImageMagick with the config options:

It seems your ruby installation is missing psych (for YAML output).
To eliminate this warning, please install libyaml and reinstall your ruby.
Building native extensions.  This could take a while...
ERROR:  Error installing rmagick:
    ERROR: Failed to build gem native extension.

        /Users/winston/.rvm/rubies/ruby-1.9.3-p392/bin/ruby extconf.rb
checking for Ruby version >= 1.8.5... yes
extconf.rb:107: Use RbConfig instead of obsolete and deprecated Config.
checking for /usr/bin/gcc-4.2... yes
checking for Magick-config... yes
checking for ImageMagick version >= 6.3.5... yes
checking for HDRI disabled version of ImageMagick... yes
/usr/local/bin/Magick-config: line 41: pkg-config: command not found
/usr/local/bin/Magick-config: line 47: pkg-config: command not found
/usr/local/bin/Magick-config: line 50: pkg-config: command not found
/usr/local/bin/Magick-config: line 53: pkg-config: command not found

The errors I got when I didn't do the symlinks:

It seems your ruby installation is missing psych (for YAML output).
To eliminate this warning, please install libyaml and reinstall your ruby.
Building native extensions.  This could take a while...
ERROR:  Error installing rmagick:
    ERROR: Failed to build gem native extension.

        /Users/winston/.rvm/rubies/ruby-1.9.3-p392/bin/ruby extconf.rb
checking for Ruby version >= 1.8.5... yes
extconf.rb:107: Use RbConfig instead of obsolete and deprecated Config.
checking for /usr/bin/gcc-4.2... yes
checking for Magick-config... yes
checking for ImageMagick version >= 6.3.5... yes
checking for HDRI disabled version of ImageMagick... yes
checking for stdint.h... yes
checking for sys/types.h... yes
checking for wand/MagickWand.h... yes
checking for InitializeMagick() in -lMagickCore... no
checking for InitializeMagick() in -lMagick... no
Can't install RMagick 2.12.2. Can't find the ImageMagick library or one of the dependent libraries. Check the mkmf.log file for more detailed information.

And seriously, you might want to consider the MiniMagick gem instead.