From the manufacturer:
Susan Bates Silvalume Crochet Hooks have the broadest size range within the Susan Bates family of hooks. Made of lightweight aluminum, each Silvalume hook undergoes a special anodizing process, which etches away all surface impurities. In addition, each hook is heat treated for strength and durability and polished for a velvety smooth finish. Our famous in line head helps reduce wrist motion for fatigue free crocheting and helps you maintain a more consistent gauge. Each hook is color coded so you can find the size you need at a glance. This package contains one 5.5 inch crochet hook. Size: D3/3.25mm. Available in a variety of sizes. Each sold separately. Imported.