Black Diamonds are used within GrizzleyWorld black market store!
Before donating to GrizzleyWorld RP on FiveM press the 'Link Account' button in settings then it will redirect you to a web page. Once you are at that page, close FiveM. Press 'Link Account' on the page it redirects you to. This will open FiveM and link your FiveM Forum account. Follow this image for help.