Mkay, basically after a lot of googling, looking up materials guides, straight up borrowing from other people's tutorials on how to make a decent diamond material, I've ended up with this:
Now, I am by no means a master at node material editing, so for the most part I just kept throwing spaghetti at the wall and seeing what sticks (basically if adding a node got it closer to looking like an actual diamond, I kept it). But try as I might, I can't get my diamond model to actually display the actual properties of a diamond (I can simulate refracting RGB colors off its sides in certain HDRIs but that's about it). This is how it looks with the current node setup:
You may say "who cares about eevee, just use cycles", but try to understand, to efficiently render a cycles scene and get a clear, transparent diamond (not even talking about adding translucency to boot), would take rendering the scene at 2500+ samples which would take a considerable amount of time and also I'm pretty sure my computer would explode.
Comedy aside, I'm pretty sure the fault lies in my geometry somehow, which just puzzles me even further (I've recreated the Tolkowsky's Perfect diamond cut with the precise angles and proportions on my model). However, adding that same material to a suzanne primitive with 3 subdivisions and a smooth shading solves the problem:
So clearly, the fault lies with the geometry somehow. Problem is, there's no way I can add a 3rd degree subsurf modifier to my gemstone model and smooth shading since then I'd no longer have a diamond. So if anyone can provide some help, suggestion for a workaround, or possible changes to my material nodes, your help would be deeply appreciated.