Onyx is a semi-precious oxide mineral gemstone that most often comes in a banded variety of black and white, but may be any number of colors including layered rings of blue, purple, and red. Pure black Onyx is the most popular, but the red variety often with yellow bands known as Sard or Sardonyx is much more rare. 

Raw (as from the ground)

Unpolished

Uncut

Polished, but Uncut

Cut and Polished