Apple’s stock rose 13-percent this month, thanks to a better than expected earnings report. It’s market value increased even more after Warren Buffet announced he bought 75 million additional Apple shares in the first quarter.

Apple is once again closing in on becoming the first company to have a market cap of $1 trillion.The iPhone is the largest driver of Apple’s business. The company sold 52.2 million iPhones last quarter, accounting for $38 billion of the company’s $61.14 billion in revenue.

Late last week, the company received yet another boost when Warren Buffett told CNBC ahead of his annual Berkshire Hathaway retreat that his company had purchased an additional 75 million shares of Apple in the first quarter, raising its total stake to $44 billion.

Buoyed by strong earnings and optimism surrounding the iPhone X, the market value of the tech giant has closed above $900 billion for the first time: $904 billion as of Wednesday. That valuation beat out even Microsoft’s climb toward a trillion dollars during the dot-com boom.

Apple would not be the first publicly traded company to reach a trillion dollars in market value. That distinction belongs to oil and gas producer PetroChina, which briefly topped $1 trillion on the Shanghai Stock Exchange in 2007.

Still, stocks can be volatile, and the optimism surrounding the iPhone X and iPhone 8 could turn to disappointment if sales of the new offerings fail to live up to expectations. Further weakening in the company’s China sales may also be cause for concern.

And unlike Microsoft in late 1999 – which hit an inflation-adjusted $896 billion – Apple has more than just domestic competition to worry about. China has become an incubator for homegrown tech giants, which have been quickly leveraging the spending power of the world’s largest middle class population.