A technique to create gold that involves a conversion of chemical elements via nuclear fusion created through incredibly high temperature and pressure.
The cold fusion theory commonly referred to in alchemy was originally discovered and established by the highest level alchemists in the Bavarian Illuminati. However, in order to make it a reality an unrealistically huge amount of magic power was necessary. The dream of advancing the theory forward was left as nothing more than academic chatter.
But, the director Adam Weishaupt has access to magic that goes far beyond unordinary limits and had no problem bringing forth enough power to make the theory a reality.
Despite the huge amount of energy he expended to create the "gold" it amounted to no more than a pitiful amount that sat in the palm of his hand.
As the name implies, "transmutation" is an act that changes the chemical makeup of an object and the extraordinary amount of energy required makes it a technique with terrible cost performance. However, being able to amass that much energy boasts tremendous power and Adam did not use it simply to create "gold" but rather as a strategic show of his capabilities.
AXZ Episode 13
The assistance from Tsubasa, Chris, Maria, Shirabe and Kirika combined with the strong wills of Saint-Germain, Cagliostro and Prelati took the form of a radiant, golden Symphogear.
It is the result of Hibiki Tachibana's blooming courage as it broke the boundaries between friend and enemy.
The fruit of her labour: Limited Model_Alchemic Gold.
Adam referred to this phenomenon as "gold transmutation," however in reality it is different from the physical creation of gold via cold fusion that he had performed before. This was a more pure form of alchemy, a true "transmutation to gold."
It seems that the crushed Spellcaster's energy which normally forms the Faust Robe became active during the violent exchanges of the final battle and acted as the catalyst. The Lapis Philosophorum that was the power source for Saint-Germain and the others does more than just cleanse impurities. Legends say it also has the ability to transform base metals into precious metals, or in other words to directly turn imperfect things into their perfect form.
In the instant that Hibiki's Symphogear turned completely golden, her body temporarily became a type of living alchemic reactor. And similar to when her body was a sacrist fusion, she demonstrated explosive power far beyond the norm and it completely overwhelmed Adam even in his full powered final form.