oh, you can tell rares by the item emblem, they are also the item rank, rank 9's and up are rare items.
You should always go for rare items with slots as those can in the end be the most powerful item you can get, regardless of what anyone else says a rank10 with a slot at maxed based is more powerful than any other item with 2 more slots.
Valhalla Quartz's I was testing them out.
The effect stacks, also they aren't worth it if your not already prepared to use them. For melee characters they are really good though, but my bow does way and by way I mean WAY more damage than a VQ'd x2 bow.