You use diamonds to purchase new weapons and upgrades from the arms dealers.
 Diamonds as Pay
When you perform tasks for the UFLL or APR you earn diamonds. The diamonds are paid in advance when you accept a mission. In Pala missions pay 20 diamonds each. By the time you reach Mosate Selao you are paid 30 diamonds per mission.
 Finding Diamonds
Usually you only find diamonds in 1s and 2s in these cases, but there are 221 total briefcases in the game so this is a decent supply of cash (as well as a good easter egg style Achievement that you might want to shoot for).
- When driving try to keep your field of view such that you can see the green blink on the dash.
- Bridges: Sometimes diamonds are underneath bridges. Don't be afraid to go for these - even though you have to jump down and can't get back up the way you came, there is ALWAYS a relatively short path leading back up to the bridge where you might have parked your vehicle.
- On top of structures: Some briefcases are on top of structures. These are always accessible by finding nearby rocks, or the husks of burnt out trucks.