How do gems built for java work? Getting error install torquebox gem

Question

I'm clearly missing something here. I'm trying to install the torquebox gems and am running into this error (on a mac):

gem install torquebox-server --pre --source http://torquebox.org/2x/builds/LATEST/gem-repo/    

Could not find a valid gem 'torquebox-server' (>= 0), here is why:
          Found torquebox-server (2.x.incremental.424), but was for platform java

I have jruby installed, version 1.6.3. It's in my path. I don't understand the above error. If you need any more info let me know.