Question

why Be does not lose its metallic luster when exposed to air​

Answer 1

Be loses its metallic lustre when exposed to air because beryllium reacts oxygen to give beryllium oxide before when beryllium is exposed to air it gets tarnished

Answer 2

When metals exposed to air and moisture for longer times then their outer surface gets oxidised and forms oxide layer ion the surface of metals. Because of this oxide layer on the surface metal appears with dull or less shiny.

When cleaned these metal surfaces with tamarind or mild acidic chemicals then this oxider layer get removed thus metal surface again appears as shiny.

Be does not lose its metallic luster when exposed to air.

this is the reason may be...antagonistically....