Onyx is a type of agate but instead of having curved bands it has straight ones. onyx comes from the greek meaning claw or fingernail because with the bands it is said to resemble the growth lines of a finger nail. Natural black oynx is rare so nearly all commercial oynx is black agate that has been dyed and treated it has a mohls hardness 6.5 and a refractive index of 1.54