Front

Back

The button looks like metal but is Black Glass with an electroplating of metal.

When Prince Albert died in 1861, Victoria went into mourning and everyone wore black.
Black glass buttons were made by the million with lots of different designs.
The older ones have metal shanks and the backs have uneven surfaces as the glass was pressed into metal moulds.
Later ones have self shanks and smooth backs as they were made by flow process.
The majority of the buttons were made in the Bohemia area, now in the Czech Republic.