CRuby's standard library is partially written in Ruby.
https://github.com/ruby/ruby/tree/trunk/lib
Rubinius' one (called RubySL) also includes some rewrites of standard libraries, that were originally written in C. And as you may know, Rubinius itself is just Ruby code to some extent.
mruby has some Ruby code, too. Even some parts of the core language are written in it.