Excellent......
Thank you!

Um. "Mexican onyx" is generally a misnomer for dyed translucent banded travertine/calcite, a fairly soft and brittle mineral, but the term "onyx" by itself refers to a type of cryptocrystalline quartz, which is much harder and more durable-- in contemporary jewelry, usually it means "black onyx", uniform opaque chalcedony; in some other fields, "onyx" is instead a type of black/white agate with fine parallel stripes.

(The word "onyx" is based on the Greek word for "fingernail" because of those fine parallel bands. I think the slippage to "black onyx" came about because the high-contrast layers of striped-agate "onyx" made it a good material for carving cameos from small slabs oriented like an Oreo cookie with dark layers on the outside and lighter layers on the inside-- but if you leave the slab uncarved it just looks uniformly black. AFAIK it's fairly standard for "black onyx" to be permanently color-enhanced via a carbonizing process, but it's still much tougher than "Mexican onyx".)