What is the world's finest powder substance that has the smallest granules but remains solid?
Clusters of just a few atoms, such as gold atomic clusters, and quantum dot nanoparticles could be considered very fine powders, though they usually are suspended in another substance to keep the clusters from agglomerating. Particles this tiny have different properties from the bulk material, both because of quantum effects (there are few particles, so there is not a continuous valence band) and because more atoms are at the surface interface than are surrounded by similar atoms in the interior.
See Quantum dot for more information.
I hope this fits your definition of a "fine powder". You might also be interested in pyrophoric iron.
BTW, ruby glass is made using colloidal, i.e. super-finely divided, gold.