The Laminate Culture colonization ship that built Torei is often characterized as a "geode" due to its robust self-reinforcing crystalline structure and protective outer rocky shell.
You can’t send a live crew on a thousand-year journey that never comes close to any stars. A generation-ship would need more fuel for life support on such a journey than it would for deceleration. The fleet they designed was effectively a big dumb intergalactic comet: machinery and gene banks and artificial intelligences wrapped in a giant geode of protective minerals and ice. Everything would just sit inactive and safe and inert until it came near the energy of a star again. --A Blagger's History of Torei