Ruby defines #clone
in Object.
To my suprise, some classes raise Exceptions when calling it.
I found NilClass, TrueCla
You can't clone immutable classes. I.e. you can have only one instance of object 42 (as a Fixnum), but can have many instances of "42" (because string is mutable). You can't clone symbols as well since they are something like immutable strings.
You can check that in IRB with object_id method. (symbols and fixnums will give you same object_id after repetitive calls)