An ammonia solution - one part ammonia to six parts water - is your best bet for cleaning diamonds. Apply it to your diamond jewelry and gently massage it by hand to loosen any dirt or contaminants. Do not use any commercial cleaning solutions or compounds that are not made specifically for jewelry - even if they say that they are "soft." They will not restore the sparkle the way that an ammonia solution or a professional cleaning done at your friendly, neighborhood jewelry store can. Be careful, though about applying this solution to other gemstones as they have other properties and not all react well to ammonia. Consult with a professional if you have questions about how to treat other gems.