Metallic luster is due to:

Metallic lustre refers to a shiny surface such as gold, silver, copper, or steel. 

Elements are made up of small pieces known as atoms. 

In the centre, there are protons and neutrons located in the nucleus. Electrons move in clumps known as clouds around the nucleus. 

The electrons furthest from the nucleus are responsible for the metal's lustre. These outer electrons reflect or reflect light. This gives the metal a shimmering appearance. The shiny appearance of the surfaces of certain metals is called lustre. 

Gold and silver are used to make jewellery because they have a perfect shiny lustre. 

Polishing a piece of metal improves its shine. Polishing eliminates particles that accumulate on the surface of the metal over time.

Hence, metallic lustre is due to the reflection of light by mobile electrons.