Diamonds have an affinity for grease. Years ago, diamond mines used a grease covered slanted table to run the mined gravel over and rinse with water. The diamonds would stick to the grease and the non diamond rough would wash away. While this method is no longer used to separate diamonds from gravel, it illustrates the effect of grease. On a finished cut diamond, the main culprit is hand lotion, as it slightly alters the refractive index and properties of the diamond and dulls it’s appearance.
A simple method of cleaning your diamond jewelry is to make a solution of a household cleaner (e.g. Mr. Clean, ammonia, simple green) and hot water, approx. 1:10. Let your jewelry soak in it util the solution is cool, then rinse under running water.
Of course, this doesn’t compare to having your jewelry professionally cleaned, but if done afterwards on a regular basis your diamonds will maintain their sparkle.