The 'json' native gem requires installed build tools

Follow the Instructions from the Ruby Installer Developer Kit Wiki:

1. Download Ruby 1.9.3 from rubyinstaller.org
2. Download DevKit file from rubyinstaller.org
   • For Ruby 1.9.3 use DevKit-tdm-32-4.5.2-20110712-1620-sfx.exe
3. Extract DevKit to path C:\Ruby193\DevKit
4. Run cd C:\Ruby193\DevKit
5. Run ruby dk.rb init
6. Run ruby dk.rb review
7. Run ruby dk.rb install

To return to the problem at hand, you should be able to install JSON (or otherwise test that your DevKit successfully installed) by running the following commands which will perform an install of the JSON gem and then use it:

gem install json --platform=ruby
ruby -rubygems -e "require 'json'; puts JSON.load('[42]').inspect"

I believe those installers make changes to the path. Did you try closing and re-opening the CMD window after running them and before the last attempt to install the gem that wants devkit present?

Also, be sure you are using the right devkit installer for your version of Ruby. The documentation at devkit wiki page has a requirements note saying:

For RubyInstaller versions 1.8.7, 1.9.2, and 1.9.3 use the DevKit 4.5.2