The faceted onyx beads (gef.) have a diameter of 8mm, in total the onyx strand is about 40cm long.
The name Onyx is derived from the Greek word "onyx" for "nail"; presumably this name, which has been in use for many centuries, refers to the surface of the polished stone, which is reminiscent of shiny fingernails. Spellings like onix or onyk are therefore not correct.