You find diamonds random either in heist or in arcade. Finish the level with the diamond.
Check out the video for a fast guide to all achievements:
Timestamps:
00:00 Intro + Successful Escape Achievement M
01:41 Out On Top, Gone in 3 Rounds, Outclassed, Getting Slick, Mastermind Achievements
02:59 Talk about multiplayer + Super Cop Achievement
04:05 Dead Weight Achievement
04:42 Hail Mary Achievement
05:08 Why Thank You
05:33 Inhuman Reactions Achievement
05:54 Clean Sweep Achievement
06:10 Last Man Standing Achievement
06:33 Oceans One Achievement
07:50 Bye Bye Birdie Achievement
08:30 Striking Gold, A Robbers Best Friend Achievement
09:01 Greed is Good, Anger Issues Achievements + Outro