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Diamond Hard is a GTA 5 Secret Achievement. It is an achievement/trophy that you can easily acquire while playing the games campaign normally.
To get this achievement/trophy, the heist The Jewel Store Job needs to be completed.
(A guide to completing this heist the "Smart" way can be found here.)
Once the heist is complete (all characters have met at the rendezvous point), you should be awarded the achievement/trophy.