Iridium

Iridium is a rare, end game metal added by IndustrialCraft 2, with GregTech adding more functionality to it.

GregTech
Iridium can be obtained from Iridium ore, or centrifuged from Platinum Dust. Iridium ore is very rare with a spawn rate of 1 ore per 5 chunks on average. In-game spawn rates will also be lowered since ores spawn randomly from Y height 1 1 to 99 and attempting to spawn ore above ground results in no ore spawned. In a biome with a Y height of 64, in-game rates average 1 ore every 10 chunks. In a biome with a lower Y height of 32, in-game rates average 1 ore every 20 chunks.

IndustrialCraft 2
Iridium can only be found in various dungeon chests. After that it can only be obtained by replicating from UU-Matter.

Thermal Foundation
Iridium can be obtained from its ore or by processing Platinum Ore. Since these ores don't generate by default, Iridium can't be obtained in normal gameplay, but can be obtained in modpacks that allow one of these ores to generate, such as FTB Academy. Other than being crafted into coins to fuel the Numismatic Dynamo, Iridium has no practical usage.