NEW YORK — The largest U.S. jewelry chain on Wednesday decided to put a ring on its much smaller rival, creating a $6 billion jewelry juggernaut with expansive reach in North America.Signet Jewelers, owner of Jared The Galleria with its “He went to Jared” slogan, and Kay Jeweler with its “Every Kiss Begins with Kay” ads, agreed to buy Irving-based Zale Corp. for roughly $900 million. Turns out, diamonds aren’t just a girl’s proverbial best friend; investors seemed to like them on Wednesday, too. News of the deal sent Signet’s stock up more than 18 percent, while shares of Zale soared more than 40 percent. The deal underscores the harsh realities of the $32. 8 billion jewelry business since the Great Recession. Sales of necklaces, rings and other jewels got hammered during the economic downturn as shoppers pulled back on their discretionary purchases. Business is slowly starting to come back during the economic recovery. But there are still a number of jewelry chains in the market, and sales are still far from their pre-recession peak of more than $36 billion in 2006. “The deal is a rational response to the imbalance of supply and demand,” said Craig Johnson, president of Customer Growth Partners, a retail consultancy in New Canaan, Conn. “There are too many stores chasing too few jewelry buyers.”In fact, analysts say the abundance of stores is a scenario that has played out in malls across the country where some of Signet’s stores go head-to-head with Zales stores. But they say Signet has fared better. Signet, which is based in Bermuda, has 1,400 Jared and Kay stores in the U.S. and 500 stores in the U.K. under the names H. Samuel and Ernest Jones. And Zale, which opened its first store in Wichita Falls in 1924, has 1,680 stores in North America under Zales, Gordon’s and other names. The company, which went through a liquidity crisis in 2009, has closed hundreds of stores and has been trying to expand its bridal merchandise. Zale returned to a profit in its most recent fiscal year, which ended July 31. More recently, the company reported that same-store sales rose 2 percent in November and December. Zales branded stores had a 4.4 percent increase in the sales for the two-month period. Together, Signet and Zale would have 16 percent of the U.S. jewelry market, according to IBISWorld, a market research firm. But even before the acquisition, Signet, with expected annual revenue at $4.2 billion in the latest fiscal year ended in January, was the largest U.S. jewelry chain. As part of the deal, Signet will pay $21 per share for Zale. That’s a 41 percent premium to Zale’s $14.91 Tuesday closing price. Excluding roughly $500 million in debt Zale had as of Oct. 31 but including the price to buy warrants held by Golden Gate Capital, a 22 percent stakeholder in Zale, the deal is worth about $900 million.