What do you want your tank to look like? Do you even want it to remotly resemble a portion of a reef? Why would you want rainbow rock? I have never heard of rainbow rock, so I can not tell you if it is safe. I doubt many people here will have experience with anything that looks that unnatural.
It looks like sandstone to me. This should be safe for a reef but I have no experience with this. It in most likely composed of cemented silica sand, if this means anything to you. The only thing that I can think of is that the dark areas may be colored by impurities of iron oxide (rust). I don't know if this is harmful to corals or not but I personally would not risk it. It (hopefully) will eventually get covered over by coraline algae anyway. I've seen it used in freshwater tanks but not marine.
The carved tufa would be fine. A LFS by me carries it. I used it on a 20 high a while back with no problems. It takes a lot longer to get any coverage to it. If you get some, rinse it for a long time. It gets dusty when little pieces break off.
