Apple stock prices surged over the $600 (£377) mark yesterday, about a month after shooting past $500. For a time, the company's total market capitalisation was more than $600 billion.

The stock price rose to $644 in the morning, and then fell back to $629 by noon. Only one other company has reached the $600 billion value: Microsoft on December 30, 1999, was valued at $619 billion. Its value is now $260 billion.

It's all the more noticeable because stocks on the main exchanges fell yet again in response to poor US job numbers last week, and continued concerns over the European debt crisis.

Apple's stock price has climbed 59% this year. A range of analysts have said the stock has been under-valued by the market, despite the company's high revenues and huge profits created by the introduction of its iOS-based mobile products.

The question is how high will the stock price, and the company's value, go? Last week, a stock analyst with Topeka Capital Markets, Brian White, attracted headlines by setting a target price of over $800 per share, and a goal of $1,001, which would bring Apple close to being a $1 trillion company. White argued that the start of iPhone sales on China's biggest mobile carrier, China Mobile, and the launch of an Apple TV set, would expand Apple's revenues and profits.

Piper Jaffray analyst Gene Munster also raised his price target for Apple's stock, but not as high as White: Munster's target lifted to $718 from $670, and others expect the price to at least top $700. Munster was quoted as saying his previous forecast underestimated Apple's "expected growth rates for the smartphone and tablet markets."

Some have speculated that the stock price surge was triggered by Apple's recent announcement that it will use some of its nearly $100 billion in cash to pay a dividend and buy back some shares. But Dirk Schmidt, writing at the Asymco blog, cites data that show the price began ticking upward faster in late 2011, and then even faster starting in February 2012, well before the mid-March dividend news.

According to one account, today's first intra-day trade over $600 came 22 trading days after the first $500 trade. That pace is faster than the 32 trading days needed to rise from $400 to $500.