I've literally knapped tons of the stuff. It was used extensively in my area (central Virginia). The material here is mostly Antietam quartzite that eons ago tumbled down out of the mountains into the rivers and washed down a hundred or more miles, being turned into rounded cobbles, which were subsequently buried and then came back to the surface over the past few tens of thousands of years eroding out into creek beds and on hillsides. Much of the parent material is light gray or whitish, but if buried long enough in the yellow and "red" (iron stained) clay soils of the Piedmont, turns that yellow-tan or rusty-tan color that is most common. Pink, purple, blue, tan and brown colors also mix in. The iron stained material if heated will generally turn red, with a slight purplish tinge.
It is tough to knap, and takes a long time to learn the nuances. It needs to be high-graded in the the extreme, with most of the material not worth picking a fight with. If you give me 10 cobbles and I get a good spall or two from two of them, I would consider that a good yield. If seeking to make a "broadspear" type knife form (like Savannah river) my success rate after making over a thousand of them is 50%.
The biggest problem with the material is that is not homogeneous, having hidden defects that do not appear until the thinning process is underway. I generally describe these as "knots" or "twisted grain" because it like trying to split apart or carve a piece of wood with such defects in it. The material is also highly inflexible, making it much easier to snap than almost all other tool stones.
It's late notice, but if anyone is in the general area and wants to see it done, there is a knap-in this weekend at the Hopper Creek group campsite in the Jefferson National forest, near Lexington Virginia. Gates open at 11 am Friday. I expect some very good quartzite knappers will be there. If life doesn't suddenly jump in the way, I plan on being there with at least 1 five gallon bucket of quartzite spalls which will be reduced over the course of the weekend to a few points, and a large pile of chips and tears of frustration.
