You've done a nice job here of fitting quite a lot of information into two-thousand-something words. Between the line breaks, flashbacks, and your mini epilogue at the end, it did not feel rushed at all. That's also thanks to your amount of detail in each scene, so that they were long enough to satisfy the reader. The flashbacks too included just enough information, but not so much that they detracted from the story.
One thing I did notice, however, was how black and white this story was. Rolf and Hannah were the bad guys, the unfaithful ones, while Luna and Neville were the poor loved ones left behind, unaware that anything was amiss. In real life, things rarely go that way. Everyone has both flaws and redeeming qualities, which is why relationships can be so tricky; maybe it's easy to make excuses for someone, or it's incredibly painful to break up even when you know you need to. I would really have loved to see more of that sort of depth to your four characters here. When a reader sees that, they are more emotionally engaged in the story, because they have to decide who to root for and the choice is not always easy. That's the sort of detail that makes a story truly complex.
I would also suggest checking out the Writers Resources section of the forums for some grammar guidelines. In many places you were missing punctuation for your dialogue, such as in this example, there should be a comma after 'sorry': "'Look I'm sorry' Rolf muttered,"
But grammar is easy enough to fix, and overall, you did a good job with this. As soon as you mentioned the legend about the emerald, I could imagine what was going to happen, so that was a very nice bit of foreshadowing there. Plus, you tied it all in at the end, giving the reader a real sense of closure. And, of course, Neville and Luna got married and lived happily ever after, which is always a pleasure to read. I was pleased that they were not set back by Rolf and Hannah's affair for too long.
I also liked the way you treated Luna. She was not over-the-top weird, nor a hundred percent normal, and you got in some of her well-known traits. For example, the Crumple-Horned Snorkack and her love of her friends, as well as her general insightfulness. That's not always easy to do. Nice job!
Author's Response: Ok I will keep all of your advice in mind, so thankyou! I think that the grammar problems are probably mostly typos, so I'll have a look soon.