Ilum crystals were the most common lightsaber crystals used amongst the Jedi until up to the Great Jedi Purge. Found in crystal caves on the Unknown Regions planet of Ilum, they were similar in structure to the Adegan crystal variants of mephite and pontite, which produced the best lightsaber blades. However, while those crystals were rare even in the Adega system, Ilum presented to the Jedi Order entire caves full of high-quality crystals. Most Jedi Padawans underwent the ritual on construction of their first lightsaber at Ilum.