This is a Dominant brand aluminum-wound violin D string. The outside cover is two flat strips wound next to each other (they’re coming off the back, top, and you can see the pair grouped together still on the string). The horizontal scratches, running the length of the string, are from the final grinding and polishing of the string surface.
Inside the top winding is another similar winding, wound in the opposite direction. They’re coming off the left, tighter on the fiber core, and don’t have the same type of scratches on them. This winding comes out the end of the string and around the ball on the lower end of the string. I always thought it was the core, but you can see here that it’s not.
The real core is a bundle of synthetic fibers, tightly wrapped within the two outer metal wraps. On a pure steel string the core would be a single strand of steel, stiff and not compressible, or in some cases a twisted wire rope or untwisted bundle.
The more-flexible synthetic core is what gives the Dominant string it’s gut-like, flexible behavior. Think of a Chinese finger puzzle: as it’s stretched, it gets smaller. If a solid rod of steel were in the puzzle, instead of fingers, it would not stretch as flexibly as if the core was something soft, like a rubber rod. The bundle of fibers in the string works similarly, as a soft, compressible core, permitting the string to stretch more than a solid steel core would, resulting in behavior more simlar to gut than steel.