One of tech's most important investors says tumbling tech stocks could kill off some unicorns

Investor Bill Gurley is taking a downturn in tech stocks over the last six weeks as a sign that Silicon Valley's golden age could soon be over.

Gurley is known for having a pretty pessimistic outlook on the tech industry, despite big investments in Snapchat and Uber. The entire US stock market had a bad day on Thursday. And in a tweetstorm last night, Gurley pointed out that some of the biggest names in tech are falling from all-time highs too. 

Tech stocks slumped across the across the board yesterday. Media stocks were hit pretty hard too, with Netflix down 7.8% at close. But this downturn isn't limited to American companies. 

This isn't the first time that Gurley has made his pessimistic outlook known. During a talk at SXSW in March, he warned that "a complete absence of fear" in Silicon Valley had led venture-capital firms to take big risks on tech companies.

Gurley went on to say that Silicon Valley's optimism could lead to the death of so-called "unicorn" companies - startups that reach a $1 billion valuation before their IPO. Those companies could face a turn in the market in the near future. "I do think you'll see some dead unicorns this year," Gurley said.

Gurley's bottom line? Tumbling tech stocks could call some inflated valuations into question.

During the recent boom, tech company founders have been encouraged to prioritize growth over profits and sustainability. That's been fine so far because interest rates have stayed low, and tech stocks and valuations have been at an all-time high. But Gurley believes a turn in the market is happening right now, meaning that investors will want to see a more viable, self-sustaining business model before they decide to back a company.

That's something Gurley believes some firms just aren't ready for.