Black granite gets its colour and glassy appearance when it's made up of around 20 percent quartz along with various other minerals, like biotite and/or black mica.
The various other minerals that are present can also leave the black granite with different coloured hues.
For example, it's common to see a black granite headstone with a brown, yellow, red, or other hue coloured finish.
Many black granites also contain small white specks which are crystal specks of feldspar.