I'd like to see what source you have that dictates the creation of the Philosopher's Stone is melting gold into a powder, because I have never heard that method in any alchemic school of thought or manuscript of the process.
The Magnum Opus creation process almost always follows the four-stage changing of color, and is listed as a seven or twelve-step process in most cases (such exemptions as Samuel Norton's fourteen-step method do exist here and there).
Where the
actual hell did you hear that melting gold so hot it becomes powder is the method of creation in alchemy's golden age during medieval times?
The only instance of ancient Egyptian alchemy that I can think of that is of any notable merit is the Emerald Tablet, but many of the associations that Tablet is said to have (most notably its relation to Hermes Trismegistus) didn't really become "big" until the medieval alchemy period.
Even so, the recipe for the Philosopher's Stone being laid out verbatim was not something that was ever done seriously in alchemy. That knowledge was believed to put one on par with the powers of the divine and was kept shrouded in stacked metaphors and layers of cryptic symbolism. "Melt some gold beyond this temperature and you have the Philosopher's Stone" not only defies the most widely-regarded processes of symbolic creation, but is not something any worthwhile alchemist of history would put to paper. There's no symbolism to it or infusion of spirituality (as alchemy's Magnum Opus was as spiritual a journey as it was scientific), there's no metaphor layering to cloud the method from anyone unable to decipher the riddles, and it just sounds silly because melting gold to make a thing that makes more gold just doesn't make much sense if you step back and really look at it.
That's something I'd expect to see a snake-oil "alchemist" claiming he has, and selling off his "recipe" for profit and making away with the money of his gullible buyers.
Also, no, the Stone isn't made purely out of gold, as gold is not something which can be considered the
prima materia, and the purpose of the Philosopher's Stone was way more than extending lifespan to immortality--you have written accounts of it supposedly granting such abilities as flight, future-vision, conjuration, teleportation, base metal transmutation, incredible healing capability, knowledge of the essence of creation, etc. etc.
It wasn't a rock that made you live forever, there is way more shit that has been attributed to it. Eternal life and gold-transmuting are simply the most well-known two, because during medieval times when most people were dirt poor and didn't live long those two things sounded pretty damn good, and thus those two properties spread like wildfire while the other alleged powers/enhancements fell to the second slide of the presentation.
I'm far from the first to say so, but these and many other seeming incongruities are easily explained by Alucard's reference to the Castle itself as "a creature of chaos". A constantly shifting (un)living being not subject to space and time as we understand them. Otherwise, the biggest plot hole would probably be why the castle looks/has a layout so different each time. Unless you suppose that there are several different castles, which aside from obvious examples like Bloodlines/Belmont's Revenge/Simon's Quest is a whole other can of worms.
I mean yeah, that's a handwave but eh XD. No reason not to keep going but be aware there's a built-in excuse for a lot of these lol.
The trouble with using the "Creature of Chaos" explanation as a literal space free from standard time constraints (which isn't really stated AFAIK, it's just said that the castle changes its form with each new resurrection), is that if the castle can have shit from outside the "normal" time period...then:
A:) who or what is getting the stuff and putting it in the castle, let alone training the denizens within how to use it effectively?
B:) why hasn't Dracula just gotten ahold of some nukes and blown all the humans to bits, since he'll just resurrect a century later anyway?
C:) why hasn't St. Germain or Aeon or whoever else relating to time travel in the CV universe stepped in to try and suppress the outrageous amount of potential for fuckery in the timeline that a literal "yeah the rules don't apply within here at all and basically anything can pop up in here whenever because why not" space would cause?
D:) why are motorcycles, chainsaws, and burgers the only things to pop up? Surely there'd be a lot of other shit to randomly appear--Harleys, Ash Williams' hand, and some Big Macs are oddly specific things to just show up.
The "time-exemption" thing isn't ever stated in regards to the Castle to the best of my knowledge, and even if it has been it still leaves way more questions than answers.