Shares of Amazon.com Inc. rallied 2.3% in morning trade Thursday, after Warren Buffett's Berkshire Hathaway Inc. disclosed that the stake it bought in the e-commerce and cloud giant was worth more than $900 million. The stock was headed for a third-straight gain--it has run up 5.0% during that time--after snapping a six-session losing streak in which it shed 7.2%. Separately, Amazon unveiled earlier Thursday its new Fire 7 tablet, for $49.99. Amazon shares have run up 27% year to date, while the S&P 500 has advanced 15%. Amazon is extending its lead over Apple Inc. as the second most valuable U.S. company by market capitalization. Amazon's market cap was recently at $942.1 billion, while Apple's stock edged up 0.3% to lift it's market cap to $880.5 billion. Microsoft Corp. was still in first, with a market cap of $989.4 billion.