Cosmetics doesn't have to have a concrete mechanical purpose. It's cosmetic. That's the point of a simulator: It simulates and throws in all the cosmetics and fancy fluff.
The stone types aren't "cosmetic". The ASCII characters and color don't give you the texture of the stone. They convey information. And this information is mostly useless.
I'll repeat the example that when I first carved a tunnel in a sand material I was worried that, being sand, it probably produced more easily cave ins. When instead I read that cave ins do not take into account the material type I was rather disappointed.
The problem isn't that nothing should exist in the game that isn't functionally different, but that stuff that you EXPECT being functionally different isn't, yet you find in the game in absurd amounts.
I'm not criticizing the specific case, but a style of development when first the variation goes in, and only later there's a worry to actually develop mechanics (if ever).
I ask you, what would entail have DF hide all types of "pointless variation" from the UI. Like: it shows you the relevant minerals, it show you the different materials you use, but otherwise the equal materials don't show any kind of color variation.
All the stone types would be still there, only their display would be altered. But on screen you wouldn't have all those rainbow colored rooms and objects. Because a bed made of stone is the same of a bed made of wood, and so give me a bed tile without coloring it fancy. Because this UI isn't about graphical fidelity, but abstraction since we are dealing with ASCII SYMBOLS.
And when you make symbols you also make sure to retain only the worthwhile kind of information, not the useless, cosmetic kind. For the cosmetic you use graphic.
AND, a good hint things are wrong in this specific case is how, in graphic tiles, a stone bed looks identical to a wood bed, whereas in ASCII you have all the garish color variations.