Kran would not eat diamonds, sillies. Diamonds are just carbon, and there are cheaper, easier to obtain, and much easier to chew forms of carbon available. Like Enki.
Don't know if they would eat mercury, though. It might make sense, as mercury readily combines with most other metals (except iron).
Other than that, mercury is used for many things. One of its uses in PS, I believe, is to purify Lumium. But, first they will need to set up evaporators and collectors for the mercury.