From Dark and Darker Wiki
Everyone that uses crystal sword realizes eventually that there isn't any point in using one until you have a purple or high-roll blue.
Every weapon in the game gets up to 80% boost to damage based on rarity from gray to unique. The crystal sword on the other hand has up to 230% difference in damage from gray to unique, it's absurd. Maybe it's time to bring the lower rarity damage up so it's useable in base kits?