AppearanceIridium Crystals are chunks of crystalized Iridium, therefore they sport the same color as it. It spawns in various sizes, but it will always have the same shape.
The crystals, when large, have a part of them visible from the underground, meaning that you can extract iridium from crystals while hidden below the Mainland
The iridium crystals hold iridium that can be extracted by a Portafab by holding E on the crystal. While Iridium is being extracted, the crystal rapidly decreases in size, until it completely disappears. A large crystal can refill a player's tank around 2 times before being depleted.
Iridium can also be pushed into crystals, increasing their size. Currently one of the only reasons to do this would be to keep crystals from being depleted by a crystal resonator, thereby over time getting back more iridium than you put in. The crystals have a limit to how much iridium can be pushed into them.
Crystals, the Ocean and the Iridium stored in the default Spawn Point is the only source of Iridium when a game starts, so the crystals are essential for starting a game (Taking Iridium from the spawn point and jumping into the ocean is possible for getting early-game iridium, however, this is a risky technique and not reccommended.)
Since crystals often spawn on the mainland, the players can take cover behind them and refill their weapons while in battle, making them a great help.
The Crystal Resonator, when placed near crystals, will create Iridium while slowly decreasing the crystal's size. It has not been confirmed yet, but most likely the Iridium produced by a crystal resonator from 1 crystal totals more than the Iridium harvested simply by a portafab from a crystal of the same size.