When first picked up, spawns a random Brickmin familiar that can be aimed and launched in a given direction when Golem attacks while they are nearby.
There are 3 different Brickmin which have different effects and behaviors:
Squares waddle slowly behind Golem. When launched, they will jump into the air a short distance before slamming down to deal 2 × Golem's Damage + 3 × Stage damage to nearby enemies and confusing them for 2.67 seconds. After slamming, Squares will "fall asleep" until Isaac walks up to one of them to wake it up.
Sharpes quickly follow behind Golem. When launched, flies directly forwards some distance or until it hits something. Hitting an enemy will inflict a stack of bruise that lasts for 6.67 seconds and deal 3 × Golem's Damage + 3 × Stage damage. Each stack of bruise increases all damage an enemy takes by 1, and is applied before the damage dealt by the Sharpe. After landing, Sharpes will return to the player.
Sproingos hop towards Golem. When launched, rolls on the ground and bounces off of anything it hits up to 3 times before stopping. They will attempt to bounce towards enemies, dealing Golem's Damage + Stage damage on hit. After they have finished rolling, Sproingos may begin to prioritize following an enemy and dash towards them every couple seconds without the player's input, which will only hit them once without bouncing. Sproingos will stop chasing enemies and return to the player when Golem "collects" them again by getting close, or until all enemies are dead.
Additional Brickmin will be gained for every 4 rooms cleared, up to a cap of 4 each floor. Entering a new floor will cause all Brickmin from the previous floor to disappear and spawn a new randomly selected Brickmin.
Notes
Brickmin do not deal contact damage unless when they are performing their attack.
When there are multiple Brickmin, each of them can only be launched one after another.
Each type of Brickmin can come in one of 3 different colors. There is no difference in behavior between each of them.
Interactions
BFFS!: No effect on the damage Brickmin deal with their attacks.
Spin to Win: Brickmin will deal 6 damage on contact every 1/3 of a second while using Spin to Win.
Trinket Multipliers: Affects the maximum number of Brickmin that can be spawned each floor, increasing by 1 for every +1 to the trinket multiplier.
Onion Rock and the Brickmin familiars are a reference to the Pikmin series.
The Brickmin familiars make various unique sounds, such as when they are thrown, which are directly sampled from the "Pikmin" in the game.
In particular, Square's effect of being slower, heavier, and slamming on the ground to daze enemies and deal increased damage is nearly identical to the Purple Pikmin in Pikmin 2.
Onion Rock is a fan suggestion from a member of the Fiend Folio Cornmunity Discord server, Moofy. The sprites for both the trinket and the Brickmin have also been designed by them.
Gallery
The 3 varieties of Square.
The 3 varieties of Sharpe.
The 3 varieties of Sproingo.
Old, unused design for Sproingo.
Initial concept art for Onion Rock, where it was originally named "Geonion".
BFFS!: No effect on the damage Brickmin deal with their attacks.
Spin to Win: Brickmin will deal 6 damage on contact every 1/3 of a second while using Spin to Win.
Trinket Multipliers: Affects the maximum number of Brickmin that can be spawned each floor, increasing by 1 for every +1 to the trinket multiplier.
Azurite Spindown: Using Azurite Spindown on Onion Rock rerolls it into 
Chunk of Gallium.
