shine at freshly cut sodium is because of

In Lassaigne's test sodium metal is used because :

A cut diamond sparkles because of its

A cut diamond sparkles because of its

A cut diamond sparkles because of its

Properly cut diamond sparkles because

Alkali metals posses metallic lustre when freshly cut because

A freshly cut piece of sodium when exposed to air for some time looses its lustre in the freshly cut portion. a) What may be the reason for this?. b) Write any two chemical equations to substantiate your answer.