From Rebirth of the Night Wiki

Onyx is the hardest natural material existing in the world(?), it is only found in the Nether and only extremely powerful alloys or materials with magic properties can shatter it enough to collect it.
It is required to slay the Wither before it is possible to acquire Onyx.
Onyx usually spawns inside a 'gas pocket' which can be seen from the surface of a biome.

Research Notes[edit]

"...certainly, onyx is fairly common in volcanic areas in the overworld, but it's no more than a semi precious stone. The real Onyx is the one found in the infinite, infernal depths of the Nether, a gem that has passed an infinitude of different process to become the hardest material ever encountered. It is unknown how a simple and very rare stone acquired such logic-defying properties. But it's thought inducing nonetheless. Appears to be mineable only after Wither has been vanquished".