How to wash dirty rock?

[DatBoi302]

I'm told I need to wash this rock, specifically at the save room located in the top right of the Support Trench. However, i'm not having any option to do so... how can I change this?

[Crash✌]

You have to clean the rock in the Save Point that has a river in it. It should be the one in the top right corner of the support trench map. It will give you a sapphire.
Take care!

[Male Pattern Baldness]

IMPORTANT
You have to wash it BEFORE beginning chapter 4. When that chapter begins, the pool is no longer accessible

[Lanzagranadas]

You can clean the Rock in chapter 4, as long as the save room was not vandalized.

I'm not sure what triggers it, but in my last playthrough I repaired both wire fences and the save room was never attacked, so maybe that's it.

[Kapdap]

Giving the prisoner in the POW camp the photo of his wife during chapter 3 triggers the damaged save room event in chapter 4. If you kill the knight in the save room you'll find out he was the prisoner and you'll also receive the widowmaker achivement.