Shares in the California-based maker of high-definition action cameras, GoPro, plunged by as much as 16 percent this week after a French motor-sport journalist claimed that a wearable GoPro camera was to blame for the severity of Schumacher's brain injuries.

"The problem for Michael was not the hit, but the mounting of the GoPro camera that he had on his helmet that injured his brain," Jean-Louis Moncet told radio station Europe 1 over the weekend.

But Schumacher's brain injury is likely to be merely a footnote in the story of GoPro's dramatic stock price evolution.

GoPro's shares have been on a wild ride since the company's initial public offering on the NASDAQ exchange in June 2014.

GoPro's cash flow and EBITDA (earnings before interest, taxes, depreciation and amortization) are healthy, with gross profits reaching $199.4 million (154.9 million euros) on revenues of $480.3 million in the six months ending in June. Sales in 2013 totaled $986 million.

But it is not clear whether the company's earnings can justify the trajectory of its share price since the IPO, at which the offering price was set at $24. By the end of the first day's trading, the price was up 30 percent, and by early October, it had more than quadrupled to a high of $98.47.

At time of publication, that price had dropped back to $76 a share. What factors have caused investors to change their opinions so dramatically about GoPro's value over less than half a year, and even from week to week?

What is GoPro really worth?

It is very difficult to value companies in high-growth sectors for which the market is not yet saturated. Wearable action cameras are a relatively new product, and sales in the sector continue to grow rapidly. It is difficult to guess how long that will continue, and impossible to know whether GoPro will be able to retain its position as the sector's dominant player.

Growth-stock valuations are not based on multiples of current earnings. They are based on what investors guess - or hope - the company might earn in the future. More mature companies' stocks tend to trade at a price that puts their stock market capitalization at 10 to 20 times annual earnings.

In the long run, GoPro's shares are likely going to trade at that rate of earnings, too. At the moment, they are trading at multiples far above those levels. A great deal of future growth will be necessary to catch up to the company's exalted share price.

But for now, GoPro's growth numbers look good.

Videos taken using the company's wearable cameras and posted online attracted more than 1 billion views in the first three months of 2014. GoPro's YouTube channel has 2.35 million subscribers and that number is rising.

GoPro's pre-IPO SEC filing estimated the company's share of the wearable sports camera market at 70 percent.

"There probably hasn't been a consumer electronics brand as dominant as GoPro has been in its category since the early days of the iPod or the iPad," Dougherty & Company analyst Charlie Anderson wrote in a pre-IPO note to clients.

But other companies have entered the action camera market, attracted by GoPro's success - and they have begun to put severe downward pressure on action camera prices.

GoPro has a first-mover advantage. However, any technology can be copied and commoditized and action cameras are no exception. JVC, Garmin, Sony, Drift, Vivitar, Polaroid and Monoprice are some of the better-known brands that now also offer action camcorders - and cheap Chinese-made knockoffs are proliferating.

The question is whether GoPro can retain its market dominance - and its profit margin - in the face of so much competition.

User community-building for market share retention

GoPro has already reacted to the growing competition by offering a bargain version of its Hero camera. But the company's founder and CEO, Nick Woodman, has a more ambitious plan for the defense of his company's dominance.

Woodman, who hit upon the idea of an action camera while surfing in Australia, is trying to turn GoPro into a hybrid - equal parts hardware maker, media company and user community.

GoPro's online presence has become a content-sharing network, with thousands of users posting videos using the company's cameras. The GoPro website includes multiple video categories, ranging from bike, music, and surfing videos to dog-mounted cameras and other critter-cam action videos.

Competitions are frequently announced, such as one for the best surfing videos posted by users. GoPro awards a daily prize, with the winner taking home samples of the company's entire product line.

The company is attempting to create a loyal, committed fan-base of users who are willing to pay a higher price for a superior product. The strategy is reminiscent of Apple's approach to marketing smartphones and computers.

Ultimately, there is no way to know in advance whether or not GoPro will be able to retain its market dominance. That makes investors' attempts to value the company's shares subject to speculation, wild guesses and rumors about the state of Michael Schumacher's brain.