One by one.
In alphabetical order. Each one says what it does and what happens if you take more than one. Which ones are worth it: the item tier list.
Benthic Bloom
At the start of each stage, it upgrades 3 random items to the next rarity (white to green, green to red). Turns your Clovers into this. Each extra one, 3 more items.
Stacking: Each copy adds the same, no cap.
Watch out: It never upgrades red, yellow or lunar items, and can take a white straight to red.
Encrusted Key
Each stage, a hidden box with an item appears, better than the Rusted Key one; it is used up when opened. Turns your Rusted Keys into this.
Watch out: The odds the game shows are not the real ones. In a group, one cache per player holding a key.
Lost Seer's Lenses
Every hit has a small chance (0.5%) to instantly kill anything that is not a boss. Turns your Glasses into this. Each extra lens, 0.5% more.
Stacking: Each copy adds 0.5%: with 200, every hit kills non-bosses.
Watch out: Useless against red-bar enemies; Armor-Piercing Rounds cover that gap.
Lysate Cell
Your special skill (R) gets one more use. Turns your Fuel Cells into this. Each extra one, another use.
Stacking: Each copy adds the same, no cap.
Watch out: It also lowers the special cooldown (the game does not say so).
Needletick
10% of your hits put a mark on the enemy that hits hard a moment later. Turns your Tri-Tip Daggers into this. Each extra one, 10% more.
Stacking: Each copy adds the same, no cap.
Watch out: It does not replace bleeds from Noxious Thorn or Shatterspleen.
Newly Hatched Zoea
Every 60 s a random void ally appears to fight for you. Turns all your yellow items into this. Each extra one, more allies and more often.
Stacking: The cooldown is 60 s divided by your copies.
Watch out: It also corrupts Pearls, Halcyon Seeds and yellow scrap.
Plasma Shrimp
Gives you a small shield and, while it is up, every hit fires a missile. Turns your AtG Missiles into this. Each extra shrimp, stronger missiles.
Stacking: Each shrimp raises missile damage (the game's +50% is not accurate).
Watch out: The shield depends on your max health: Shaped Glass shrinks it.
Pluripotent Larva
An extra life: if you die, you come back, but every item with a void version gets converted. It is used up. Turns Dio's Best Friend into this.
Stacking: One extra life per copy; used up on revive.
Polylute
25% of your hits strike the same enemy with lightning, up to 3 times. Turns your Ukuleles into this. Each extra one, 3 more strikes.
Stacking: Each copy adds the same, no cap.
Watch out: Very low proc coefficient, like the Ukulele.
Safer Spaces
Blocks one full hit; works again after 15 s. Turns your Tougher Times into this. Each extra one, faster recharge.
Stacking: Each copy cuts 10% off what is left of the cooldown.
Watch out: A guaranteed block against a big hit, but not void fog damage.
Singularity Band
When you land a very big hit (over 4 times your damage), a black hole pulls enemies in and then damages them. Every 20 s. Turns Runald's and Kjaro's Bands into this. Each extra one, more damage.
Stacking: Each copy adds the same, no cap.
Tentabauble
5% of your hits root the enemy in place for 1 s. Turns your Chronobaubles into this. Each extra one, likelier and longer.
Stacking: The chance adds less each time; the duration adds the same.
Voidsent Flame
Your first hit on an unhurt enemy makes it explode and hurt those nearby. Turns your Will-o'-the-wisps into this. Each extra one, a bigger blast.
Stacking: Each copy adds the same, no cap.
Watch out: Like Crowbar: in a group it is hard to land the first hit.
Weeping Fungus
You heal 2% of your health per second while sprinting. Turns your Bustling Fungi into this. Each extra one, 2% more.
Stacking: Each copy adds the same, no cap.
Other rarities: Common items · Uncommon items · Legendary items · Boss items · Lunar items.