Apple CEO Tim Cook took the extraordinary step of reassuring investors that Apple's business was doing just fine even amid the cratering of global markets.

And it seems to have had an impact on Apple's stock. After Apple had cratered Monday morning, with share prices falling below $100 pre-market, Apple made a huge comeback. The company is now up by more than 2% after it was down as much as 6% hours ago.

In an email exchange with CNBC star Jim Cramer, Cook said iPhone sales had accelerated in China over the past few weeks:

Markets globally are tanking based largely on things happening in China. The Chinese economy has looked weak. With China being the second-largest iPhone market, investors were naturally worried about the state of the iPhone, which Cook addressed in his letter.

"I think it's a classic fear of the unknown," Piper Jaffray analyst Gene Munster said in a previous interview with Business Insider. "In this case specifically it's the fear of what's happening in China. Investors appear to be concerned about what the impact is from China and if that ends up translating to more broader concerns or a slowdown from consumers."