Silver is like living organ, it reacts to the surrounding environment, for example humidity or sea water or body salts can cause a reaction what make silver darker. But we have a good news, silver can be always cleaned and repolished! In domestic conditons you can use soft brush and toothpaste and later on polish it with soft cloth. But be aware that it can scratch the surface of your silver jewellery, so we actually recommend professional product (also safe for gemstones). See it from:
HERE. Don’t use liquids, where you have to dip the whole piece of jewellery, you can damage the stone or remove oxidised surfaces.