Did you ever wonder what onyx is, exactly?

 

I'm not sure what prompted it, but I abruptly became concerned that I did not know what onyx was, exactly, other than a type of gemstone.

First, do not confuse it with "oryx".  That's what this is:

What is onyx, then?  According to a website that should know, onyx is "a variety of microcrystalline quartz, called chalcedony." That's also known as agate. It is most famous for being black, like below, but it doesn't have to be. 

Following right behind the basic black is banded onyx, which is black and white, which polishes up quite nicely.  There's also green and red onyx.  Apparently it's also possible to make fake onyx with dyes and regular agate, which I don't think is the right thing to do.

As my final note, it can also be faceted like a diamond, but if it's black, that's hard to see.