The Diamonds are featured in Grand Theft Auto IV,The Lost and Damned and the recently revealed The Ballad of Gay Tony wherein they are involved in many scenarios also involving the Protagonists, Niko Bellic, Johnny Klebitz and Luis Fernando Lopez as well as many others. Detailed below are the many different scenarios which take place involving the diamonds and the different people that gain possession of them each time. The diamonds are the main purpose which led to the Impossible Trinity achievement.
In the The Lost and Damned's storyline, Gay Tony and Luis Lopez are seen to attended a meeting with the ship's cook in order to obtain the diamonds. This deal however, was interrupted by Johnny Klebitz and several members of The Lost, on the orders of Ray Boccino.
After the diamonds have been collected, Luca and his crew attempt to steal the diamonds for themselves and plan to move to Las Venturas, but are stopped and are killed by Niko.
Later on, again on the orders of Ray Boccino, Niko teams up with Johnny, though not for the first time, and attempts to sell the diamonds to Isaac Roth and Mori Green, members of the Jewish Mob. The deal is interrupted by Luis Lopez, who appears to achieve his goal of stealing the diamonds (although it is unclear whether he obtained them during this deal). This deal is seen played out from two different perspectives in Niko's point of view and Johnny's point of view.
Later on Niko and Packie kidnap Gracie Ancelotti on the orders of Gerald McReary in an attempt to trade her for the diamonds at a later stage. Gay Tony and Luis Lopez agree to strike a deal to trade the diamonds for the safe return of Gracie at an Oil Refinery on Charge Island. Soon thereafter, the deal gets underway and seems to be going smoothly until Bulgarin appears, claiming that the diamonds were his before Gay Tony and Luis stole them. Following a shoot-out, one of Bulgarin's men grabs the diamonds and escapes up onto one of the metal walkways surrounding the refinery and holds them over the railings when he is cornered by Niko and Packie. Fearful for his life, no matter his decision, He decides to drop them over the railing into a passing garbage truck which is headed for the dump and with that he is promptly killed. This is the last time the diamonds are seen.
The last mention of the diamonds however happens when listening to Weazel News. It is said that a homeless man found them in a landfill, and were valued to be worth $2 million. 100 people attempt to obtain the diamonds before the mayor makes the decision to give them to the homeless man. The homeless man then speaks of his plan to sell the diamonds and use his money to open up a gun shop and a liquor store in Vice City.
The Diamonds will appear in the upcoming DLC, The Ballad of Gay Tony. Possibly involving on the major storyline of the game.
Note the following missions are sorted in chronological order of the timeline.