By default, RVM will try to download and install a binary distribution of Ruby. If no version is specified, the latest stable version will be used. As of this writing, RVM chose a binary distribution of 2.0.0p247. But what if you're using Ruby 1.9.3 still? Or (you should really think about upgrading if you are) 1.8.7? RVM can do that, referring to version numbers instead of "ruby," RVM will assume a version number of MRI. So, here's how to install Ruby 1.9.3.
$ rvm install 1.9.3
To see which versions are available (as well as which interpreters, the MRI interpreter is at the top of the list), use the command rvm list known.