Californium: £18.7m per gram
Californium can be used to start nuclear reactors and treat cancers. It is also found in metal detectors. But at $27m per gram, you'd have to be finding some pretty precious metal.
However, there is, actually, one thing that is more valuable than even this...
#4 & #7... Not sure what I am missing here.. But it seems that if I grind my solid gold bars up into a fine powder I can increase it's value by 700%.. from $28 to $200 a gram??