Rarity is shown by the color of the item when you are unlocking it.

There are 3 rarity levels: Common, Rare, Epic.

Common (blue): 

Common items are the most likely to be found. These are typically lower-level items.

Rare (green): 

Rare items are harder to find and are infrequently found. Rare items may have better levels or abilities and appear more visually interesting. 

Epic (purple): 

Epic items are the most unique and scarce items. These are likely to posses higher levels and awesome effects.