Well, the FDA has trouble with the concept of "Natural" as applied to food labelling, so why should glass be any different? "Natural" is a label that people apply to suit their needs.
Your list of categories are not mutually exclusive, so right off the bat, your labels are ambiguous. There are natural metals, plastics, etc.
The only obvious conclusions is that glass is NOT a metal, since it's a poor thermal and electrical conductor, and it's NOT a "plastic" under the common definition that a plastic is composed of "organic" molecules, although, obviously, there are many plastics that don't have components that come from "organisms" and so wouldn't fit a narrower definition of what "organic" molecule means.
Glass can be considered a ceramic, because it fits the broad category of a material that is formed by heating and subsequent cooling, and is considered to be an amorphous ceramic. Likewise, one could claim it's "natural" because it's composed of naturally occuring materials, which is often a definition that slides by FDA scrutiny.
TTFN
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