If its a piece of plain platinum jewellery, it can be generally washed. It can also be washed if it set with diamonds, garnets, sapphires, amethysts, rubies. Do not wash if it contains emeralds, aquamarines, opaque stones (e.g. pearls).

• Fill a small plastic bowl with luke warm water.
• Add a dash of colorless, perfume free washing up liquid into the bowl. (you can also use jewellery cleaners) If the piece contain diamond, you can soak it in gin to get rid of the grease
• Place the jewellery into the water, and leave it for a few minutes
• Use a very soft brush (e.g. baby tooth brush) to remove the dirts
• Rinse the jewellery in plain clean water
• Lay the jewellery on a dry tea towel
• Polish the jewellery generally with a spectacle cleaning cloth or chamois leather. Do not use normal towel or cloth as those may catch on the stone settings. Ironically, most people lost stones from a ring is after cleaning, so always be careful.