In chapter 7: Invitation Only, make your way through City Hall part 2 and head towards the mayor's office.

Kill off the zombies that block your path and once inside his office, check the safe under his desk. You'll obtain "Diamonds".

Make your way through the rest of level and exit through the sewers, onto the next part of this mission.

Once you obtain the checkpoint after spawning in the sewers, turn right back around and head back through the door leading to City Hall.

Careful, once you make your way through the first room and pop back out into the workbench room, a Thug will spawn out of nowhere right behind you.

Make your way back to the mayor's office, and open his safe again. More "Diamonds".

Repeat untill satisfied.