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All of the Detect X spells (in addition to Message) have the following text in their description
The spell can penetrate barriers, but 1 foot of stone, 1 inch of common metal, a thin sheet of lead, or 3 feet of wood or dirt blocks it.
In this context, what does RAW have in mind for "Common Metal"? Are there any other classes of metal defined anywhere?
As others have already noted, there's no RAW that I know of that define the idea of common metals beyond the statements in the spell descriptions. Since we're talking about detection, and possible deterrents to detection, it seems logical to me that the "commonality" of said metals is more intended to define the scope of the detection powers, rather than a purely scientific mechanism based on density or other qualities. So I'd address this as a function of game balance between magic and world economics.
In other words, common metals are whatever metals are commonly (and therefore, perhaps cheaply?) available in your setting. This would mean that walls or containers that resist scrying/detection would either be bulky (due to the required thickness of the "commonly" available materials) or very expensive (since presumably thin sheets of lead or more exotic metals would be harder to acquire). I would also extend the comment about lead to apply to other "non"-common metals.
For instance, I wouldn't classify precious metals such as gold or silver as "common" for these purposes, despite the fact that we "see" a lot of it in fantasy settings, because 1) their very value is predicated on their rarity, and 2) using them to line walls or containers is a fairly costly proposition, which I believe is in keeping with the intent of the detection spells text.
Likewise, anything that is widely used for weapons, tools, household goods, or construction, such as brass, iron, tin, pewter and copper would be common for the purposes of the spells.
And of course, anything exotic, such as mithral, adamantine, meteoric iron, etc. would be un-"common", as well as pricey/hard to work with.
It basically means “not lead (or similarly dense metal)” – which of course isn’t all that specific.
It’s safe to say that iron, steel, and mithral (typical equipment metals) fall in that category. Adamantine, I’m less sure. Bronze, copper, and brass are probably common as well. Gold is denser than lead so it probably fits in the same category as lead.
Ultimately, the rules are simplistic and don’t make any attempt to be exhaustive. It’s pretty much up to the DM.
