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 
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
An oscillator’s output goes up and down. They can be sine waves (called harmonic oscillators), or any kind of wave. The ones used in digital circuitry make square waves.