A quote from previous discussions;
On every "Rockhound Board" in the country, the most often questions posed about identifying specimens, refer to the varieties of cryptocrystalline quartz.�

Varieties of cryptocrystalline quartz are called Agate, Jasper, Chert, and Flint,etc. There are some that are no trouble to identify and others that look like a mix of several varieties. Color has a lot to do with it but in the end, is unreliable. The most important question is always, "How was it formed?"That looks like a fragment of an agate to me, but I might change my mind if I saw the whole original piece. Pretty material.
I am partial to black and white patterns so the Bear Creek Agate is very attractive to me. Too bad it seems there are so few of the good Bear Creek agate nodules. You have to cut quite a few for a couple decent ones.
And here's more reading;
http://www.quartzpage.de/agate.html