It is always recommended that you collect your diamonds from the base to avoid having them stolen by other players.  PvP losses occur quite frequently, so be sure to collect them by the hour.

To earn more diamonds,, do the trial missions. Trial missions are located on the map and are on all 3 regions. There are two kinds, one with Tesla Rifles and one with Heavy Guns. By completing these missions, you are able to amass great amounts of diamonds. 

Another way to acquire diamonds is fight other players in PvP and steal their diamonds.

Note: You may also lose diamonds even if they're accumulated at your base, so be sure to use them before they get stolen!