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The diamonds in the bottom left corner indicate the card's rarity. Converting the rarities to their physical card game equivalents: 1 diamond is a common, 2 diamonds is an uncommon, 3 diamonds is a "rare", 4 diamonds is an "ultra rare", 1 star is an artwork/illustration rare, 2 stars is a special illustration/artwork rare, 3 stars is the immersive cards which are unique to pocket, and a crown is a hyper rare.
How rare they are is in the exact order I typed them, going from most to least common. In TCG Pocket, even a "rare" is not guaranteed in every pack, though.