I seem to have forgotten the source of the Immortal Essence Stones. They are only a hard currency, because they can be converted into personal Fruity Immortal Essence/fuel formations/can be used as a material for refinements, which would imply that the total amount in existence decreases at every moment. Except if there is a source/production point for them. Can anybody remember it? Do rank 8's simply produce them en massé? Are they mined, even though nobody ever has had the idea to dig underground in this world outsides of earth trenches? This question is bugging me more than it should...
They are distribuited in the market by Heaven Court, no one else can produce them and no one know how they are producing them. They also can't be found naturally. Spoiler: How they are made It's already been revealed how they are produced in the raw but I am not gonna explain it in depth. To make it short it involves the use of a unique Immemorial Legendary Beast that has a special characteristic to be able to produce them. Spoiler: Another Spoiler That Beast already died long in the past, so all the Immortal Stones that they are using and spreading (there is a reason as to why they are spreading them in the market) are just a large stock pile they had before the beast died.
I just want to add: Spoiler: How they are made The Immemorial Legendary Beast produces Rank 8 Immortal Essence Stones (which HC didn't sell). Every Immortal Essence Stone on the market is Rank 6 one, which was produced by HC itself.