Silver coldplate standard on the top line of Aquacomputer Kryos Next
on some of their older models also, btw.
Also, if you would take an IHS to a silversmith , and tell him you want the same thing made out of as pure silver as possible (while still strong enough) , i am pretty sure (s)he can do that for you
However, price will most likely exceed the above waterblock. Not because silver is "soo expensive", because it really isn't that much different in pricing as copper (have you seen copper prices recently?) ... but because of the hours which will go into handcrafting a silver IHS.
And in the end, all that for a net gain of 2 to 5°C
You see, the copper IHS is not the barrier here.. the thermal barrier is the TIM. You'll gain more from eliminating the TIM as from using "exotic" metals.
Lap both the CPU and the Waterblock to such a perfect finish that they stick to each other and you can do away with TIM. Or just apply TIM and wipe it off so there is only a film so thin that its only a sheen.
All that being said.. delid and go for direct/naked. Although in reality this requires a shim between die & block. But you can get a shim of the right thickness made out of silver also.. after all its only a square piece of silver 0.8 or 0.5mm thick. And guess what.. silversmiths have things like that laying around, only needs to be cut to the correct size
One more thing... silver is a far/high as you can go. Gold is to "soft" and diamond is too "irregular". That said, diamond can be "grown" with its cells oriented such that thermal conductivity is optimal; just like Graphene can be constructed as such... one drawback thou to "grown" diamond & graphene... the pricetag.
footnote: the vials of graphene floating in water on ebay... snake oil, a scam. They have no practical application unless you posses very very very tiny tweezers and an electro-microscope to correctly orient the particles by hand. And IF you do have that equipment.. please present yourself to Marvel Studio and apply for the role of evil mastermind in the next Avengers movie.