This is gold-plated material on a silver base. The "585" hallmark corresponds to the gold plating. Therefore, an acid test is unfortunately not sufficient.

What stands out here, once again, is the craftsmanship of the clasps and the weight of the jewelry.

The photos clearly show that the clasps are offset from one another; this is not usually the case with high-quality gold jewelry, as it compromises comfort.

Furthermore, the locking ring is open. It can come loose easily and is usually soldered in place. That is not the case here.

Last but not least, the chain is hallmarked three times—once on each clasp and once on the locking ring. This is highly unusual.