Diamond

Diamond is a metastable allotrope of carbon where carbon atoms are arranged in the face-centered cubic crystal structure called a diamond lattuce (hence its name). In large part because of it being a non-renewable resource and its relative rarity compared to other materials, it was often sought out. Diamonds that haven't been cut or processed yet are called rough diamonds. In some cases, cremated human remains can be used to form diamonds as a way of memorializing them, called memorial diamonds.

In 1984, the Private Force Diamond Dogs often procured rough diamonds during their missions as a means of raising their GMP. Diamonds were also among the precious metals that Diamond Dogs often mined, processed, or even procured during missions that they used to either raise their budget or, in some cases, for R&D development. Some rough diamonds of various sizes were also found on Mother Base. The Diamond Dogs themselves had been partially named after the root word for Diamond, adàmas, to represent their unyielding attachment to something.

Shortly after DD (at the time still a puppy) was brought back to Mother Base, Ocelot told the PF commander, Venom Snake, that DD needed to be trained, and compared the puppy to a diamond in the rough regarding his usefulness in the future.

The Mbele Squad, shortly after its capture, was sent to the mines by the Buta and Rogue Coyote, where they mined diamonds. They later attempted to give diamonds to Snake when he arrived to kill them under the orders of the Mbele general, with his proceeding to fake their deaths shortly afterward. The Contract Forces of Africa armored column commander also kept a stash of diamonds at Nova Braga Airport, with his later exposing the location of the stash to Snake after the latter interrogated him, as the commander believed him to be Big Boss.

After an incident on the Quarantine Platform involving a second outbreak of the vocal cord parasites where Snake was forced to kill the infected due to there being no cure, their cremated remains were turned into memorial diamonds at Venom Snake's order, in order for them to provide "a shining light to [Diamond Dogs'] brothers in arms, even in death."

Behind the scenes
Diamonds are a collectible item in Metal Gear Solid V: The Phantom Pain. They were generally used to bolster the player's GMP count, and can be classified as either small or large (designated as "Diamond (S)" or "Diamond (L)"). The former has a net worth of 10,000 GMP, while the latter allows for 100,000 GMP.