Diamonds Are a Girl's Best Friend is the final mission for the McReary Family in Grand Theft Auto IV. In this mission, Niko Bellic and Packie McReary arrange a deal on Charge Island to exchange their hostage, Gracie Ancelotti, for a ransom of diamonds. The mission later appears through Luis Lopez's point of view in Ladies Half Price.
Niko meets Packie and Gracie in a car on Frankfort Avenue in Northwood. The trio head to the Charge Island Sewage Works, and once there, they meet Tony Prince and Luis Lopez. Niko and Packie offer to give Gracie back to her family in exchange for diamonds that Luis stole in an earlier incident. While Gracie is successfully returned to her family, the deal is interrupted by Ray Bulgarin, whose men temporarily steal the diamonds. Niko and Packie engage in a prolonged gunfight, eventually confronting one of Bulgarin's men as he holds the diamonds hostage. The man throws them onto a passing dump truck out of spite, trusting neither Bulgarin nor Niko and Packie to let him live. Packie kills him, and he and Niko lament that they have no ransom and no reward as they head back to Packie's Ma's house.
In order to complete the mission the player must:
- Go to Packie's car in Northwood
- Drive to the Charge Island Sewage Works
- Go around the back to the exchange point
- Take out Bulgarin's men
- Make your way across to the walkway
- Take down the reinforcements
- Take out the last of Bulgarin's men
- Follow Packie
- Take Packie back to his Ma's house in Dukes
On the radio and in The Liberty Tree an article mentions a homeless man becoming rich after finding a bag of diamonds in the garbage. The man said he was going to Vice City to open a weapons business. However this is retconned in The Ballad of Gay Tony where in an updated version of the story states that he found them in the trash in Meadows Park (as seen in the closing cutscene of the mission Departure Time) in Luis' side of the story.
- Packie telling Niko to take the "low road" while he takes the "high road" is a reference to The Bonnie Banks o' Loch Lomond, a traditional Scottish song.
- After the diamonds are dropped into the Biff, most of the vehicles encountered on the streets of Liberty City until the end of the mission will be Biffs.
- Niko's line when Bulgarin's last surviving henchman drops the diamonds into the Biff ("fucked up, as usual" in Serbian) is the same line he says in the introduction of The Lost and Damned.
- The mission's name is a reference to the song of the same name from the 1953 film Gentlemen Prefer Blondes.
- The Vincent used in this mission is dark green, but in Ladies Half Price it is black.
- Similarly, Bulgarin's Marbelle is black in this mission, but in Ladies Half Price it is white.
- After this mission, a news report claims Jerry Kapowitz found the diamonds, which can be seen happening in Departure Time in TBOGT. This creates a continuity error, as Departure Time is said to take place shorty after the ending of GTA IV.