What Is Luster And What Affects It?

from my understanding luster or the lack of it is caused by the dies age and deterioration. my definition of a coin's luster is the frost that appears from striation and ware of the die causing a cart wheel effect which can be easily destroyed by handling and cleaning while a non lustrous coin with original surfaces comes from a new or newly polished die that has minimal to no deterioration at the time the specific time the coin was struck.

SuperDave is completely correct. It essentially boils down to surface roughness. Proof fields are the smoothest and reflect light consistently. Frosty fields are rougher so they scatter light in more directions. I believe that fields can be so rough and scatter light so much they seem less lustrous/bright.

As coins age, the metal surface oxidizes and you develop a film on top of the bare metal surface that the coin was manufactured with. That will often not reflect light as well and that factors into the change in luster as coins age.

These are all things that occur on a very small scale (microns and nanometers) over a long period of time. You can really get into the chemistry and physics of it all if that's up your alley.