Rubber is a natural product which is best kept dark and dry.
Rinse with lukewarm water with a bit of detergent , blot dry and dry to air.
A deep black shine is obtained by rubbing with silicone spray
(wear gloves !) and whipe the exess fat with a soft cloth.
A matte grey color is created by powdering with talcum powder,
afterwards rub with a soft cloth.
 (These treatments are only suitable for pieces without textile fabrics !)
Always keep your rubber jewelry seperate from your other jewelry to avoid discoloration.