"king" of materials indeed- this really would represent the "king" of materials if it existed! There isn't a true elastomeric o-ring material which will survive this temperature- 600 F is the rough limit for elastomers. Pseudo-o-rings made out of metals do exist, but don't work exactly the same way.
Greetings,
Metal o-rings are an option. I used them successfully for a high temperature Inconel heat exchanger. At 800F you are probably talking about silver coated Inconel or Inconel X. There are many items that must be considered in order to make a reliable seal (e.g., surface finish, optimal o-ring cross-sectional diameter, vented or non-vented o-ring, thermal expansion of flange and bolting, etc.). Metal o-ring are not forgiving like elastomeric o-rings.
Do a Google search on "metal o-rings" and contact manufacturers. They can help make sure you consider all relevant design considerations.
Here are the Helicoflex line of energized seals that may be suited for your application. I have used several configurations of their seals in various high temperature and pressure applications as well as some high vacuum and high temperature applications
Download the Product Manual as it gives a very through explanation as to seals work and their applications.
