Gold Jewellery Cleaning Acid. The original acid pickle salts for gold, silver & copper. Don't use a polishing cloth (or any kind of polish), ammonia or baking soda to clean your gold plated jewellery.
I think maybe the stain is from the soldering? You can also safely clean silver or platinum jewelry in an ultrasonic machine. Dry the jewelry gently with a soft cloth.
Pouring the acid into a coffee filter the acid passes through the filter leaving the gold behind, save the acid do not dump it down the drain.
Although there is no reaction between chlorine and pure gold, the main effect stems from various alloys, as most diamond jewelry is designed with a combination of gold and platinum alloys.
This technique is not as gentle as using soap and water. Lay it on a dry towel with the stone facing down so the water drains out of the setting. Normally gold is acid tested by rubbing the item on a special stone (to get a "streak" of gold) and then dropping acid onto the stone.