Progressing through Stardew Valley will require you at some point or another to have a large amount of Iridium, one of the rarest ores in the game. Getting to the point where you can reliably mine Iridium will take some time to reach, but once you do you can reliably get more, even if it isn't always in the biggest quantities. Iridium is especially useful for upgrading your tools, as it's required for the final upgrade.
Where to find Iridium
Iridium Nodes have blue spikes poking out of them.
The primary place to find Iridium is from Iridium Nodes, which are found in the Skull Cavern. Iridium Nodes are relatively uncommon on Floor 1 of the Skull Cavern, but the chances of them appearing increases every floor you go down. Using Staircases or Bombs, you can very quickly descend into the Skull Cavern which will drastically increase the amount found. You will want to upgrade your pickaxe to Iridium as soon as possible, otherwise breaking any rock in the Skull Cavern will take multiple swings.
In addition to the Skull Cavern, there are a few other ways to get Iridium. Iridium can appear in both Omni and Lava Geodes, both of which can be found in the lower floors of the normal Mines. The issue here is that the rewards are random, so you aren't guaranteed to get Iridium from these Geodes. It can also appear for sale at the Traveling Cart and in fishing Treasure Chests, but again, these are random, so there is no way to reliably get Iridium from these sources.
