Gravium is a very rare mineral and has the property of emitting negative energy as soon as it is irradiated with neutrons, making the Interstellar Drive possible.
With Gravium, the Interstellar Drive cause the fabric of space ahead to contract and the space behind it to expand. The ship rides on this wave inside a region of flat space and doesn’t move within this bubble but instead be carried along as the region itself moves due to the actions of the drive.
The effect was already discovered in 1972 by a Russian charwoman while dusting in a military science lab. She started accidentally a neutron source which irradiated a crumb of the yet unknown Gravium on a table.
The result was that half of the building was destroyed and this event left the Russian scientists and military completely baffled.
The effect was only rediscovered in the year 2179 by bored Moon Base scientists while "studying" old game blogs in the Internet archive database
Gravium is a glowing mineral, can be found in smaller accumulations of ore lumps and is very easy to distinguish from other ores because of its bright blue color.
Gravium is used up like fuel so you should always carry enough Gravium to return safely back to your base!