If you try to install a ruby gem you are likely to receive the following error:
ERROR: Error installing caldecott:
ERROR: Failed to build gem native extension.
mkmf.rb can’t find header files for ruby at /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/ruby.h
It clearly mocks about the missing ruby.h file. To overcome this issue you need to have XCode installed and there goto “Preferences/Downloads”
Download the “Command Line Tools”. Afterwards the gem installation should succeed with a breeze.