The correct sentence is usually using the preposition : parler de beauté.
However in some contexts, you might decide to drop the preposition. In my opinion (it is just the way I interpret it when I read it or hear it, I have no other proof), the main interest of dropping the de is to emphasis the object of the verb, and draw more attention to the object than to the verb.
Parlons affaires : The objective here is to make business, not to talk.
Causons politique : More than just chatting, I want to communicate about politics.
Elles parlent chiffons (idiom) : They are talking about futile stuff. What I want to communicate is not that they are talking, but
that what they are doing is not interesting according to me.
Please notice that you can only do that if the object complement is very general. You cannot say something like
Parlons Jonathan or Causons ma voiture.