I agree that there is significant overlap and that a second word was not really necessary.

Klingon doesn't have words because they're really necessary; it has words because Klingons have a word for something. Languages have redundancies and overlap. This isn't a problem. Why should English have both shiny and glossy? I find a Klingon word for glossy much more useful than a Klingon word for yodel or protein.

However, being in this situation now, I would say that the difference is the smoothness. Glossy = shiny + smooth. All glossy things are shiny, not all shiny things are glossy. The blade of my katana is shiny, but not glossy. The lacquer on the scabbard is glossy.

I would further suggest that only a coating can be glossy and not an uncoated object, but I haven't fully explored and tested that concept.

Anthracite coal is an example of something glossy but having no coat. Some kinds of silk fabric are glossy without being coated with anything.

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SuStel
http://trimboli.name