Heridium is a Silicate element, commodity, and crafting material in the game. It is used to craft Antimatter and some upgrades. It can be found on planets and mined from tall rocks. These are quite common. You can fly close to the surface with your starship to spot them. They give hundreds of Heridium per rock. It can also be bought at any space station from the trade terminal and merchants. They usually sell 20-50 Heridium (you need 50 for crafting Antimatter).