I'm not sure it even makes sense to talk about a core of Ruby in the same sense. The semantic core of Ruby is very light, but while a small Lisp is recognizable Lisp, a "small Ruby" without the weight of the Ruby syntax and a whole slew of runtime libraries, is not really likely to be recognizable as Ruby.

What makes Ruby is all the sugar.

E.g. MRI 1.8.x has a parser alone that is about 6000 lines of Yacc with C actions and workarounds for Yacc limitations.

Your best bet for a semantic core of Ruby would probably seem closer to Smalltalk than to Ruby...