Endurium is a blue Ore, that can be refined into better ores.
It is found on the X-1, X-2, X-3, and X-4 maps, but high concentrations of it can be found in X-3, and X-4. You cannot find this raw material anywhere in Upper Maps. Further more, it has a base ammount of 15 credits. However, if the player has honor points, the worth is increased (maximum increase is double). Also, if the player has negative honor, its worth goes down.
It is the 2nd best Ore material found in space. Worth more than Prometium, but less than Terbium. These can be sold at your local Space Station (located on the X-1 map). You may also sell your cargo in the Uppers home map (X-8).
So if your cargo space is full, sell it at a Space Station, or if you wish to stay out in space a little bit longer, Refining your cargo can be a good idea as well. This turns the 3 basic materials into the more expensive Prometid and Duranium, and allows more space for cargo.