The new power
The next night, Shirou was back at the forge. Beside the flame itself was the starmetal he had started work on last time. He once again lifted it into the flames and while it heated up he looked around the forge.
The three metals were still on place on the shelf, by the wall were any tools used in normal forging, around the ceiling danced the lights from Aestus Estus. Shirou took all of these facts in before returning to the fire and taking out the red hot starmetal.
He knew he had to be careful, one wrong blow would warp the metal's ability to channel essence and force him to start over. Before each hammer blow he would look at the blade. He would trace out the patterns of essence with his vision enhanced by his now semi-divine nature, he sort for the places whose magic wouldn't warp when he struck.
By following these instincts, the blade began to take on shape. At some point during the forging process the light that named the blade "Aestus Estus: The Original Flame" had rejoined the metal. It had directed the essence flows in new designs. The only way to forge the blade without ruining the blade was precise shaping based on the flows of essence. Superficially the blade now resembled the form of the original blade but it was now longer and looked like steel with the colourful reflections that starmetal brought instead of pure red.
The main difference that could be noticed if one had both blades to compare was that the original had a form that reminded Shirou of waves. The reforged blade still had the waves but they were much smaller. If you were to pick the blade up and you knew the original blade, you would also notice that the blade's center of mass is off compared to the base sword. Shirou had bought it more in line with the blade to make it easier to wield in his style.
The remaining lights clustered around him then, as if asking to be allowed to join with the blade. Shirou allowed them to.
"Aestus Estus: The Original Flame / a hand-crafted crimson sword / Created in a reality marble / made from starmetal / the ultimate instrument / Wielded by Nero Claudius / Owned by Shirou Emiya / The key of the theater"
The text was now all white.
Shirou picked up the blade, it's metal was slowly becoming more red to match it's legend but he could still see the multi-coloured reflections within the blade. He took it outside and swung it about a bit. It was a perfect blade, In his hands it didn't overswing. He could stop it in a moment if he needed to. The blade cut only as much as it needed to and no more.
In his name he renamed it, the blade's legend accepted the new name.
"Aestus Estus Reficio; The original flame reforged"
He planted the sword into the mud, grass and flowers sprouted around it.
Shirou woke up with the sun.
He knew he wouldn't be called on today so after leaving a note, he left camp to go a mile or two south.
There he stood, and he traced.
Judging the concept of creation. It was once Nero's blade, now it was his
Hypothesizing the basic structure. Reduced waves for better air resistance, larger blade for more power.
Duplicating the composition material. Starmetal, the unknown metal from this world.
Imitating the skill of its making. He had the skills, he had reforged it after all.
Sympathizing with the experience of its growth. He had only his own memories to work from. That was enough.
Reproducing the accumulated years. He saw the blade cooling and him placing it into the earth.
Excelling every manufacturing process. The blade was new, and yet old. It had been forged in the soul and from there it was called.
In his hands the blade appeared. He felt it's weight, heavier than it had felt inside his reality marble. It was a two handed weapon so he had expected weight.
He swung it around experimentally. Like before even if he decided mid-swing that he didn't want to make this cut, the blade would stop still. He wondered then if the reverse was true when fighting to kill?
Eventually his practice had finished and the blade faded away.
He began making his way back to the camp, before he walked very far however he saw someone.
He struggled to remember her name, it was something like Marge.
She looked at him agasp. He realised she probably saw his practicing.
Slowly she asked "Where did you find such an artifact?"
Shirou tilted his head, he hadn't expected her to care but he answered "I made it."
The woman stamped once on the ground "Impossible, that camp doesn't have access to starmetal! If you truely forged it, where did you get the materials?"
Shirou again realised something was wrong, the only was to describe it was that she was acting 'Out of character' he didn't really know her so he didn't know how she was acting different but he felt it.
The woman winced "Sorry, it doesn't matter." she quickly turned and began heading back to the camp in a hurry. Shirou walked back at his own pace.
He travelled to Rai's tent and left another note. It said something like "This is Shirou, tell Nereth I have recovered a bit of my power."
He would have delivered the message in person but he didn't really want to ask where her tent was unless he needed to.
Cloaked Blessing gave her observation reports to Yu-shan via prayer before lying back in the bunk arranged for her.
Somehow Emiya had gotten his hands on a starmetal artifact, he claimed to have made it himself but that was unlikely as he had no materials.
She had included within the report a request to know if any starmetal fell in this area and had not been collected so she hoped that revealed more but the mysteries of Emiya grew with each day.
End of chapter.
So yes, the new reality marble is a way to forge/reforge magic weapons within his soul.