What’s the most expensive or valuable substance you can think of? Gold? Diamonds? They’re on the list. But the most valuable substance by weight is currently Californium. Yes, antimatter is technically worth orders of magnitude more, but since it is still highly impractical to produce, store, and use it other than for research (using current technology, it would take 10 billion years to make a gram), Californium tops the list.
Click the infographic below to see a larger, more legible version.