I usually avoid Onyx personally because often I do not think that as a stone it holds much personality. However--once I began researching it for this blog I found how terribly wrong I was.
While onyx in it simplest form is a varient of a type of quartz--I often only see it is the black as coal stone seen below:
However--onyx is a fascinating and ancient stone, used by Egypt in houseware construction, used by Romans and the stone is even mentioned in biblical references. And further, its sister stone Sardonyx is often misunderstood and misrepresented--probably unintentionally though not all banded stones that seem to be prevenlantly sold as Sardonyx are as hard and lasting a stone as true onxy and sardonyx.
I have used onyx in jewelry designs myself--my mother terms it a protection stone as are most black stones. My sister Jenny particularly prefers it, and as such I usually do not put Onyx on my site as almost all that I purchase goes to her as a gift.
Sardonyx on the other hand is sometimes sold as Jade in Asia--as my husband brought me many jade housewares from his tour in Korea, which I was grateful for, but still told him promptly they were in fact Sardonyx. Nonetheless, upon further research I see that since it was from Pakistan it was likely one of the lesser stones labeled wrongly.
This a a red banded sardonyx, stunning in person and sometimes sold as a simple banded agate. Because of the banding in these stones a piece of perfection is impossible to achieve and the uniquness should be emphasized as every piece will be in fact, one of a kind--no matter how many stones from the same deposit one purchases.
Here is an interesting answers.com listing of Onyx--as you can see they call it a chalcedony.