Pretty, but edgy, vintage black pressed glass brooch in the shape of a bow.
Probably from the 1930s but maybe older.
It's 5.5cm long, 1.5cm wide and has a slightly rusty pin - easily cleaned up.
I found a box of black glass jewellery in an old warehouse I cleared and many of them have CZECHOSLOVAKIA impressed on the back or printed on their display cards. Czechoslovakia was formed in 1918  but the region of Bohemia and the Czech lands had been famous for glass production for hundreds of years (stating in the 1500s)