Onyx is a variety of chalcedony or microcrystalline quartz which is formed in the gas cavities of lava. Generally, most onyx has a parallel banding structure (as opposed to agate which is more irregular). Sardonyx is a variant of onyx in which the coloured bands are sard (shades of red).
As a healing stone, sardonyx is called the stone of the senses, improving sensory perception and strengthening immune system. The stone encourages one to be more adventurous, fearless, and willing to take on challenges.