Rubeum is a rare Element in No Man's Sky obtained from worlds in Red Star Systems.
Rubeum has no uses in crafting at this time.
Where To Obtain
Here is a planet where Rubeum can be found that only requires the first 2 Portal Glyphs to dial:
It is a Low Bio-Hazard Planet with Medium Flora.
Fungal Clusters also grow on this planet enabling the harvesting of Fungal Mould used in many crafting recipes.