So, you say it is a rule, but it doesn't work like you being able to tell me where the rule is? Or what it says.
Just like half plate says that it consists of shaped metal plates.
So what armor would you say someone is wearing if they have shaped metal plates that cover most of their body, heavy leather boots, a helmet, and gauntlets?
Because half-plate doesn't come with a helmet or boots, or gauntlets. So if my character is wearing those items, do I get to increase my AC to 18 instead of 15+dex? I mean, if the rule is that half-plate is only defined by what is in the description, then if I have more armor on, I must be wearing something else right?
It is in the same place than the rule that states that wall of fire cannot be made out of anything other than fire.
So, it is this?
"A spell is a discrete magical effect, a single shaping of the magical energies that suffuse the multiverse into a specific, limited expression." Because, this is one of those things that say that a wall of fire creates a wall of fire. "Specific, limited expression"
We also have this
"Each spell description in Chapter 11 begins with a block of information, including the spell's name, level, school of magic, casting time, range, components, and duration. The rest of a spell entry describes the spell's effect."
There is also this from Tasha's
"Just as every performer lends their art a personal flair and every warrior asserts their fighting styles through the lens of their own training, so too can a spellcaster use magic to express their individuality. Regardless of what type of spellcaster you’re playing, you can customize the cosmetic effects of your character’s spells. Perhaps you wish the effects of your caster’s spells to appear in their favorite color, to suggest the training they received from a celestial mentor, or to exhibit their connection to a season of the year. The possibilities for how you might cosmetically customize your character’s spells are endless. However, such alterations can’t change the effects of a spell. They also can’t make one spell seem like another—you can’t, for example, make a magic missile look like a fireball."
So, it seems we have quite a few rules about why a wall of fire can't be a wall of water (which FYI is also a different spell) can you show me anything similar for half-plate being made out of different materials, or are we still in "it doesn't work like that" territory?