Rubedo

Rubedo or 'Gold' is the single most valuable material in the world. It is a material with supernatural qualities and the purest among all precious metals. While it isn't the rarest material in the world, it sure is the most sought-after, especially because it cannot be produced via alchemy - but it is needed for almost every alchemist process as either a component, ingredient or as a material for tools and containers.
The most precious quality of this unassuming, dead-red metal is its pureness. It is incorruptible and deflects any impurity on a physical and spiritual level. When you try to mix it with another material, it will always separate into its own entity again. And no matter where it is found, it will not have any resonance. It's dual in nature; which means it appears the same body, shape and form in the meta realm as in the outer worlds and can travel through the veil unharmed, unbent and unchanged. Spirits, ghosts and humans can see, feel and touch it and perceive it in the same way, which is a very rare quality to have for any material. 
And all this leads to the reason why rubedo is the most sought-after metal in the world: It can contain, touch and endure any type of aether without changing resonance, form or shape. Because of these unique qualities, rubedo is used to line tools and containers for alchemical processes, it's used in many rituals as catalyst, amplifier or abherent, depending on the type of craft. Further, is is of invaluable importance for gunsmiths, tailors, armor makers and warfarers as it remains unimpressed by any form of aetherical power. This means it cannot be altered, damaged or corrupted by Logos-alterations or Nephesh spells. Bullets or weapons lined with gold will ignore any supernatural shield or armor, and skillfully crafted artifacts lined with gold  will be able to hold immense aetherical power without breaking, amplifying their energy source many-fold. Armor lined with rubedo on the other hand will stretch and adjust even to the dimensional change of a changing logos - but never break. Gold-woven cloth will obscure a Sinner's aura and aether resonance which is of great importance for combat and undercover military units and a must-have in any standard issue Grey Man or Helel Ben Sha'har agent's uniform.
Rubedo is the most precious among the 'Big Five' elements in the Magnum Opus of alchemists, even Prima Materia as gold can be used to filter and aether to a point were it becomes 'raw'.
In the Crimson Empire, Rubedo is a sacred metal, used in ceremonies of worship of Their Majesty the Lightbringer and their apostles.

Properties

Material Characteristics

Rubedo is a precious metal which is relatively soft, but of high durability. It' has a profound sheen in white over yellow to reddish tones depending on purity. The most precious rubedo ore is Crimson Rubedo - it is riddled with thick, blood-red veins and has a dark, rich golden-red shimmer.

Physical & Chemical Properties

It doesn't oxidize and can be easily formed into a variety of shapes at relatively low temperatures by a skilled fine-smith.

Geology & Geography

Gold is mined all across Europe, but is especially plentiful in the newly discovered Black Continent, Behemothia. The Romans have a monopolistic approach in mining rubedo on that continent due to their colonies there. So the Crimson Empire is currently the nation with the most gold.

History & Usage

Everyday use

Rubedo is in high demand with every nation of the known world because of it's supernatural qualities.

Type

Metal

Value

Very valuable.

Rarity

Moderately rare, but not evenly dispersed among the landmass of the civilized world.

Color

Gold - from white over yellow to reddish hues.

Common State

Solid

Rubedo is the most precious among the 'Big Five' elements in the Magnum Opus of alchemists:

• Prima Materia - Raw, pure, uncolored aether without resonance. Can be used to shape and form any other material.
• Rubedo - Gold in it's varying qualities and forms; the only material that can hold the fleeting, raw aether without polluting it.
• Citrinitas - A pale, ugly, yellowish metal that's dull in color as a ore and not much more when furnaced; it's very rare and holds a terrible secret only skilled alchemists can unlock.
• Albedo - The Bones of Angels; a porous rock-like, bleak material found in the earth. It's a fossil fuel with supernatural quirks and qualities.
• Nigredo - The Blood of the Devils; a thick, black tar-like substance residing deep within the earth. It's a fossil fuel with supernatural quirks and qualities.