Rails won't install

Hi folks.

ruby 2.0.0p247 (2013-06-27 revision 41674) [universal.x86_64-darwin13]

Rails won’t install:

Building native extensions. This could take a while...
ERROR: Error installing rails:
  ERROR: Failed to build gem native extension.

   /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby extconf.rb
mkmf.rb can't find header files for ruby at /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/include/ruby.h

Gem files will remain installed in /Library/Ruby/Gems/2.0.0/gems/atomic-1.1.14 for inspection.
Results logged to /Library/Ruby/Gems/2.0.0/gems/atomic-1.1.14/ext/gem_make.out

Any advice appreciated.

Cheers

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