What will aqua regia not dissolve?

The Limits of Aqua Regia: What Resists the King's Water?

Known for centuries as one of chemistry's most potent mixtures, aqua regia—a corrosive blend of nitric and hydrochloric acid—earned its name by dissolving "royal metals" like gold and platinum. However, despite its legendary reputation, this powerful solution is far from omnipotent.

While gold, platinum, and mercury readily succumb to its chemical attack, other noble metals put up a fierce resistance. For instance, iridium and silver cannot be dissolved by aqua regia. In the case of silver, a passivating layer halts the process, though both silver and palladium dissolve quite well in plain nitric acid.

Other platinum-group metals require very specific conditions to react at all. Ruthenium only dissolves in aqua regia when oxygen is present in the environment, whereas rhodium will resist the solution entirely unless it is first reduced to a finely pulverized powder. Ultimately, aqua regia remains a remarkably effective chemical tool, but its legend has very distinct boundaries.