https://www.pbs.org/newshour/nation/rescue-crews-rush-to-north-carolina-mountain-towns-cut-off-by-helene-floodwaters
Quick google search shows that China has its own Quartz mines, so I'm not sure if those are unsuitable for certain semiconductors or if the use of "world" here just means 
It is very pure. You can use any quartz, but then you have to purify it first, and it is expensive.
really? you have to do that for like, everything you mine. does this NC one just have such an amazing deposit that it makes other options uneconomical?
Kinda? Western NC has been a mining hub for forever and they aren't short on quartz at all. You could do it other places but I imagine the startup costs and the relatively poor margins on that kind of manufacturing make it unattractive, at least in America.