If Diamond had more drums in (like Girl on Fire for example), I would find it acceptable to call it an R&B song, but in its current state, I think it's a bit of a stretch. Someone Like You is closer to R&B than Diamonds I think.
Rihanna does have some R&B songs, but this isn't one of them.
I think the changes in the chart could be a good thing for urban music though. Let's face it, the genre is dead at the moment, and this will encourage artists to make songs that can crossover again. And will hopefully put an end to this stupid "dance song for pop, hip hop song for urban" releases that everybody does. I'm fed up of it.
THAT is a reach. SLY is clearly adult contemporary. It is not R&B.
It is not R&B. 
