Aes Master
Details
Mastering Aes asks you to have complete control of the object you're working with; to bend and shape it however you want, and to alter how it behaves and looks. It's a highly impressive and sought after skill, but it’s difficult to master.
Your character must show the ability to alter a mineral or metal however they want. Size, shape, color, state of matter: any characteristic is free for the taking of an Aes master. Do they play with multiple materials, or are they picking one single base and creating various unique outcomes? Are they interested in the artistic, artisanal design? Or is this just a proof of concept?
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If you asked a layman what's the best skill of an Aes master, they would probably reply with the ability to turn any one material into a completely different one. Labeled the skill of Alchemy by some, its high skill floor doesn't deter learners from practicing and honing their Aes mastery until perfection.
From one base, show your character creating various new materials. Do they pick a common rock, or something else? What items are they transforming it into? Precious gems, valuable metals, unique rocks? Do they have difficulties with the transformation into some materials compared to others? How many do they make before feeling like they reached the peak?