Trump spikes 30% in one prediction market after FBI letter on new Clinton-related emails

Donald Trump spiked more than 30 percent (Unknown: TRUMP.16) in one prediction market, following Friday's news that the FBI is reviewing new emails related to Hillary Clinton 's personal email server. She gained back some points after initially dropping 10 percent in the same market.

Contract at Predict It track the market's chances of a particular candidate winning the presidential election next month. As Trump's contract rose and was trading at 26 cents, the Democratic presidential nominee's Predict It contract (Unknown: CLINTON.16) traded lower at 77 cents.

FBI Director James Comey sent a letter to lawmakers Friday to supplement his previous testimony on the investigation related to Clinton's emails.

"In connection with an unrelated case, the FBI has learned of the existence of emails that appear to be pertinent to the investigation," Comey said. "I agreed that the FBI should take appropriate investigative steps designed to allow investigators to review these emails to determine whether they contain classified information, as well as to assess their importance to our investigation," he added.

Predict It is a real-money political prediction market, or a stock market for politics. The Victoria University of Wellington project allows users to anonymously buy or sell shares of different political "contracts" to analyze market perceptions of a given event.

—CNBC's Gina Francolla contributed to this report.