Your jewellery is made of 925 sterling silver and all pieces have a surface treatment. These surface treatments will fade over time because of contact with clothes, hard surfaces, sweat, perfume, hairspray, air pollution and others.
The black oxidized layer fades with everyday or very frequent wear. The gold plating is 2-3 layers of 18 carat gold that gradually wears off, giving the appearance that it has faded. Everyday or very frequent wear, may speed up this process, especially on items like rings and bracelets, which have more frequent contact with surfaces.