Subadamantine describes:

• A. A type of cut
• B. Surface appearance with slightly less sharp reflections than adamantine
• C. A metallic luster
• D. A glassy varnish

Answer: (B)

Understanding luster helps describe how light behaves at the surface of a diamond. Adamantine luster is the very bright, extreme reflectivity that diamonds can show, with sharp, mirror-like reflections. Subadamantine describes a surface appearance where those reflections are still bright but a bit softer or slightly diffused compared with true adamantine. It’s a near-adamantine level of luster, used to note the small drop in sharpness while still being very bright. The other options don’t fit because this term isn’t about the cut or facet shapes, it isn’t metallic in character, and it isn’t described as a glassy varnish—diamonds either show adamantine or a slightly softened version of that luster, which is what subadamantine captures.