Rarity

A Rarity is a type of item. Most kinds of rarities play a role in upgrading gear.

Acquisition

A variety of rarity items are available in Spiral Knights, and can be purchased from the Supply Depot in exchange for energy. Kozma also sells them since talking to her opens up the Supply Depot. Much like other upgrades, they are tradeable and may be purchased from other players through the auction house or direct trade. Keep in mind the rarities you find in the clockworks are not tradable however.

They are located in your arsenal, under the Rarity section.

Evo Catalyst

A contained charge of highly unstable energy that can be synthesized by Battle Sprites. In order for a Battle Sprite to advance and level up, a required item will need to be fed to the Battle Sprite. At certain levels, the required item will be an Evo Catalyst, allowing the sprite to evolve. Without the required item, the battle sprite will not level up, no matter what or how many materials the player feeds the sprite.

A battle sprite will require an Evo Catalyst at levels 14, 49, 89, 94, and 99 before advancing a level. The catalyst used at 14 (Evo) will unlock the second skill, the one used at 49 (Advanced Evo) the third one, and the other ones (Ultimate Evo) will let you upgrade the sprite skills to their ultimate versions. Evo Catalysts are tradeable and can be sold in the Auction House.

Evo Catalyst

The most basic form of Evo Catalysts, this 2-star rarity item will be required to unlock a second Battle Sprite skill. The sprite will evolve into greater forms and learn new skills. It can be purchased in the Supply Depot or Kozma for 125 energy.

Advanced Evo Catalyst

This 4-star rarity item will be required to unlock a third Battle Sprite skill. The sprite can evolve into even more advanced forms with even more powerful skills. It can be purchased in the Supply Depot or Kozma for 250 energy, or you can get it by doing the mission "Built to Destroy".

Ultimate Evo Catalyst

This 5-star rarity item will be required to unlock an ultimate Battle Sprite skill. The sprites can evolve into their ultimate forms and harness the power of their ultimate skills. It can be purchased in the Supply Depot or Kozma for 525 energy.

Fire Crystal

A rare, crystallized formation of heat used to forge higher levels of weapons and gear. Fire crystals may be purchased in stacks in the Supply Depot or Kozma, crafted via the recipe, acquired via missions or found as loot in treasure blocks or in Forge Prize Boxes. They must be used within the Forge in order to level up your items after acquiring the requisite amount of heat. Your choice of the number of fire crystals to use with each upgrade will have an influence on the success of the upgrade and the likelihood that the gear gains special bonuses, such as double level up or special. The heat you earn in the Clockworks remains yours, regardless of the success of the upgrade.

Only Fire Crystals purchased from the Supply Depot can be traded and can be sold in the Auction House.

Cracked Fire Crystal

Though cracked, it still glows ever so faintly. This rarity item is required to level 0-star weapons and gear. It can be found as loot in the Clockworks or purchased from Vatel.

It may also be crafted using the Cracked Fire Crystal Recipe.

Dim Fire Crystal

The heat within flickers like a tiny candle. This rarity item is required to level 1-star weapons and gear. It can be found as loot in the Clockworks or purchased from Vatel.

It may also be crafted using the Dim Fire Crystal Recipe.

Warm Fire Crystal

The heat within burns strong enough that it feels warm to the touch. This rarity item is required to level 2-star weapons and gear. It can be found as loot in the Clockworks, it can also be purchased in the Supply Depot or from Kozma in 50 unit packages for 85 energy.

It may also be crafted using the Warm Fire Crystal Recipe.

Glowing Fire Crystal

The heat within burns bright enough to light a darkened room. This rarity item is required to level 3-star weapons and gear. It can be found as loot in the Clockworks, it can also be purchased in the Supply Depot or from Kozma in 50 unit packages for 175 energy.

It may also be crafted using the Glowing Fire Crystal Recipe.

Shining Fire Crystal

The heat within burns like a star shining in the night sky. This rarity item is required to level 4-star weapons and gear. It can be found as loot in the Clockworks in Stratum 5 and 6. It can also be purchased in the Supply Depot or from Kozma in 50 unit packages for 350 energy.

It may also be crafted using the Shining Fire Crystal Recipe.

Radiant Fire Crystal

The heat within is wild and mysterious, shining brighter than any natural fire. This rarity item is required to level 5-star weapons and gear. It can be found as loot in the Clockworks at depth 25 and deeper. (Note that for the sake of loot, Danger Rooms and Shadow Lairs are deeper than their marked depth) It can also be purchased in the Supply Depot or from Kozma in 50 unit packages for 700 energy.

Spark of Life

Sparks of Life are rarity items that contain a powerful charge of pure energy that instantly revives you or a fellow party member. Reviving with a Spark consumes one spark of life, revives the player to full health and creates a large energy blast. Heat is not lost during this process.

Sparks of Life can be earned via missions, found in treasure blocks as rare loot drops, or purchased in bulk at a discount price of 200 energy for 10 sparks in the Supply Depot or Kozma. Sparks of Life cannot be purchased while in the Clockworks, however, they can be purchased individually to revive via a popup —presented to players who have fallen in combat without an emergency revive— at a rate of 50 energy per unit.

Notes

Rarity items were introduced to the game on release 2013-07-30.

They seem to be manifestations of energy, able to function much like Energy used to (especially in crafting and reviving) prior to their release.