It's been almost a year since we made two major changes in the JCK Jewelers' Directory Issue. First, we changed the publication date from mid July to late December. Second, we made many improvements in the product listings. We're very pleased to know, thanks to a just-completed readership study, that our subscribers appreciate both changes. So far we have only the first highlight information from the study, conducted for us by Globe Research Co., an independent market research company in Melville, N.Y. The study reveals that two out of every three jewelers consider the Directory either "extremely useful" or "very useful" &endash; a truly remarkable response considering that the survey covers all retail jewelers, not just JCK subscribers. Again, close to two-thirds of all those responding to the survey say they find the expanded listings easy to use and more useful than those they r