Oddly-shaped Opal

The  is an item in Risk of Rain 2, added in the Survivors of the Void expansion.

After 7 seconds of not taking damage, the holder gets the buff, increasing armor by 100 (+100 per stack) until they take damage again.

Math
The amount of damage that the Opal mitigates can be quickly found with $$Opals/(Opals+1)$$. One Opal reduces damage by 50%, two by 66.66%, so on and so forth. This value differs slightly depending on the character's base armor, however.
 * Because armor stacks hyperbolically, having more than a few Opals quickly becomes redundant; for instance, having 5 Opals gives 83.33% damage reduction, while having 6 gives 85.71% damage reduction.

Tips

 * This item pairs exceptionally well with Safer Spaces, allowing the player to completely negate 1 incoming hit and then greatly mitigate another.

Version History

 * Survivors of the Void
 * (Undocumented) Added New Item: Oddly-Shaped Opal