Libra

Libra is a passive Zodiac item.

Effects

Grants 6 Coins, 6 Bombs and 6 Keys.
Continually balances Isaac's primary attributes - damage, range, tears, and speed (has no effect on luck, tear height, or shot speed). This effect is NOT one time only - Libra will rebalance Isaac's attributes whenever they are changed by an item, effect, etc. including temporary effects such as the The Book of Belial.

The balancing is based on Isaac as the character himself, and he gains no bonuses or penalties to any stat from simply having Libra on its own.
The balancing formula for Libra values damage a little lower than other stats. This means that the standard damage up effect will actually grant less damage than a standard speed up or range up effect will.
Stats are capped before Libra's balancing is applied. This means that if speed is maxed out (2.0) already, a Speed Up effect will not increase any stats at all.
Since tear height is not involved in the calculations, items such as Mini Mush or Kidney Stone that increase tear height but lower range are mostly detrimental.

Blood Clot / Chemical Peel: The stat rebalancing doesn't affect the modified tears.
D8: D8's stat changes depend on which stats end up with which multipliers and their base values. Range has a high weight and is uncapped, and thus can result in a massive boost if it scores a high multiplier. Damage is also uncapped but, on the other hand, has a low weight.
Ipecac: Isaac rapidly fires small Ipecac bombs at a lower base damage.
Taurus: Isaac's damage and range increases as speed increases. However, if Isaac's base speed is not high enough, the invincibility never activates.

Soy Milk: Isaac's Tears will be reduced (still faster than the normal maximum without Soy Milk) and his other stats will be increased significantly. Without other items, the damage will be higher than Polyphemus and speed will be maxed. Because of the Range increase, Isaac's tears fire at a height that lets them travel over obstacles like rocks or poops.

The symbol of the scales is based on the Scales of Justice held by Themis, the Greek personification of divine law and custom.
Libra gives 6 bombs, 6 keys, and 6 coins, a potential reference to the Number of the Beast (666).

PC F4C8 CPWM (First floor treasure room)

PC G3VX ADL3 (Treasure room adjacent to spawn)

PC FJGQ 16DZ (First floor treasure room; second floor treasure room contains soy milk for synergy.)

PC LME3 1YAY (Treasure room adjacent to spawn)

PC LVK4 LE8W (First floor treasure room)

PC 39EQ E7Q2 (First floor treasure room two left of spawn; treasure room two right of spawn contains Soy Milk for synergy) (Hard mode only)

PS4 PEBQ QMMW (Treasure room adjacent to spawn)

PC 3GMP EP3E (First XL Floor treasure room adjacent to spawn; Soy Milk in treasure room of next floor) (Hard mode only)

PC RXPJ 0GCX (Treasure room adjacent to spawn)

Switch CQWB 0CFV (First floor treasure room)

Switch T2AE MZW1 (Treasure room adjacent to spawn)