Quantum-crystalline armor is unique armor equipped to the Sun Crusher. It made the Sun Crusher nearly indestructible. Examples of how strong it is include that the Sun Crusher once rammed right through the bridge of an Imperial-class Star Destroyer without taking any damage, and it also managed to survive for extended periods inside both the gas giant Yavin and even the supernova caused by the explosion of the planet Carida's sun.
The armor's main weakness is that it is far more vulnerable to focused energy blasts, as a glancing blow from a Death Star prototype's superlaser caused severe damage to the Sun Crusher, with the implication that a second hit would have destroyed the craft. Though extraordinarily powerful blasts are required, it can be presumed that a blast from the finished Death Star (or Death Star II) would have instantly destroyed the Sun Crusher. The armor is also very expensive to make, with the armor on the fighter-sized Sun Crusher costing as much as the first Death Star.