Shares of Signet Jewelers (SIG -23.32%) were taking a dive after the company gave a disappointing update to the key holiday quarter and cut its forecast for the fourth quarter.As a result, the stock was down 25.6% on the news as of 11:47 a.m. ET. Signet comes up shortSignet, which is the world's largest retailer of diamond jewelry, said that same-store sales were down 2% through the first 10 weeks of the quarter, which ended on Jan. 11.The company now expects same-store sales to decline 2% to 2.5% for the f ...