Rarity is one of the factors of any item in SkyBlock. An item's rarity serves as a good description of its worth and usability. In the case of Armor, Weapons, and Accessories, rarity also affects how strong/costly a Reforging will be. Rarity is not to be confused with an item's Drop Chance (although they are often related).
There are 8 rarity levels of items:
Common items are usually relatively easy to get from the start of the game and not very valuable, except for when sold in bulk. These items range in value from a seed, worth 0.5 coins and considered very easy to get, to a Diamond Block worth 72 coins and considered somewhat harder to get for an early-game player.
Uncommon items can be hard to obtain at the start of the game but become more common as a player progresses. These items range in value from a Diamond Pickaxe, worth 12 coins without enchantments, to an Enchanted Blaze Rod, which can sell for about 400k coins to the bazaar.
Rare items tend to be hard to get even mid-game, but quite valuable, and often necessary in order to reach the late-game. These items range from pieces of the Miner's Armor Set worth approximately 1,000-4,000 coins each, to the Enchanted Diamond Block, commonly sold for 200,000+ coins in the Bazaar.
Epic items tend to be very valuable. Examples include late-game level armor and weapons like the Scorpion Bow, which is one of the most powerful bows in the game, which commonly sells for millions without enchantments, and the Tier IV - Tier VII Perfect Armor pieces worth ranging from 4,000,000-8,000,000 coins when sold in the Auction House.
Legendary items are some of the most valuable items in the game. This includes the Aspect of the Dragons, currently considered one of the most powerful swords in SkyBlock due to its damage capability, and is worth millions. Another example of a Legendary item is the Tier XII Perfect Armor, which has the highest defense - and with that, EHP - in the game.
Mythic items are items that you can get by using a Recombobulator 3000 on a Legendary item to make it a Mythic one, or a level 9 enchantment book such as Infinite Quiver 9.
Special items are usually limited-time items or items that only a select group of players currently own. This includes the Game breaker which is given to players who reported a game-breaking exploit or glitch (like dupe glitches and server crashes) and New Year's cake, which is given at the end of each in-game year by the Baker. See more Special items.
Very Special items are Special items that have a Recombobulator 3000 applied onto it. Although you can do this, it is more recommended to use a Recombobulator 3000 on prestigious armor and weapons.