I don't think I would have described anthracite coal as glossy, but OK. What if I say it like, "having a smooth and shiny coating or looking like it has a smooth and shiny coating"?

I don't think the perception of a coating really has anything to do with it. One can have glossy lipstick, and when it's put on the lips one can say the lips are glossy because they're coated with the lipstick, but the lipstick is glossy whether it's on the lips or not.

Glossy silk isn't coating anything. Even if you wear it as a literal coat, nobody would say you look glossy in that coat; they would say your coat is glossy.

There are some plastics that are glossy all the way through, not just on the outside. When you cut into them, the new surfaces are also glossy.

It's just that gloss is measured from the reflected light of a surface. Coating or the perception of coating isn't what's relevant; reflectivity and smoothness matter.

-- 
SuStel
http://trimboli.name