The Lucidians were a species of space-faring explorers. According to the Tenth Doctor, they travelled by using crystalline stardrives, using a green crystal also known as Lucidian, which "magnified their cognitive-affective functioning”, turning their intense emotions into a source of fuel, which they used to explore the universe.
The Lucidians had lond-since gone extinct by 1852, although in this year the Tenth Doctor discovered some of their crystal in an orphanage on Earth. (PROSE: The Shape on the Chair)