Consider that oxidation changes over time and can be related to humidity of air, menses, or other factors that might vary the acidity of your epidermis.
Polishing your jewelry quite frequently and keeping your skin dry can reduce this chemical reaction.

The best way to take care of your jewelry more frequently is to rub the metal with a leather cloth without any product. 

When you want to give your jewel extra care, I recommend using high-quality anti-tarnishing products, like the ones here below for bronze or silver polishing. I'm not an affiliate of these brands, I just have been using them for years on my jewels with great results.

In specific, if you're looking for a product to take care of your Giardinoblu jewels, I recommend you Hagerty, copper brass and bronze polish.

There are several good products available online at various price ranges. They're the best way if you want to take back your jewels at their original brightness in just a few minutes from home.