Material is not appearance alone
Colour, gloss and texture are valuable observations but cannot independently identify geological source, species, alloy, pigment, dye or treatment.
Composite objects need layers
Support, surface, attachments, inlays and modern mounts may carry different materials and histories. Record boundaries before reducing the object to one word.
Display belongs in the record
Glass, lighting, backing, distance and inaccessible sides shape what a visitor can verify. Date those conditions and keep official object records separate.