Can pieces of 1-3/4" flagstone just be laid on top of decorative rock? Thanks.
No, is the short answer although it depends whether it is to be walked on or not. There is also the question of levelling that will be difficult and what you will put in between the flags. Explain a bit more and perhaps there is a less black and white answer.
How big is the "decorative rock?
Generally, the recommendation is to lay the flags on a properly prepared base and then surround with smaller, decorative rocks. In AZ, there is no need to mortar a flgstone walkway, loose or dry laid is sufficient, but for the leveling/safety issues INK alludes to, you do not want to lay directly in top of decorative rock.
Here is a link that might be useful: laying flagstone pathway