10* rarity items are unique to enemies that are level 56 or higher. As of now, not all enemies have them yet. 10* rarity items tend to have unique effects that require being upgraded to function well, which require Raremetal Shards. Raremetal shards can be obtained by breaking down 8, 9, or 10 star items (8 = 1 9 = 2 10 = 3, respectively, at a 50% rate).
There are two currently known limitations regarding 10* items.
You must be at least level 50.
You cannot have more than 3 10* items equipped at once.
All the (currently known) 10 stars can be found below.