Mercury is liquid, not crystalline, and is reflective. Diamond is crystalline and transparent. Delocalised electrons cause reflectivity, and this has nothing to do with crystalline structure.

And here they found yet another trade-off. While the best light reflection (nearly perfect reflection) came with a multi-layered, 3D structure, that added significantly to the weight. By contrast ...

Seeing no reflection: Two pieces of aluminum nitride, a semiconducting material that can be used in light-emitting devices, reflect different amounts of light. The piece at the top reflects 12 ...