Sleek designs pose in an equally modern 5th Avenue setting; smooth cabochon-cut stones, most from South America, glow in pale wooden display cases. The designers make notable use of semiprecious stones such as citrine, tourmaline, and topaz. Subway: E, V to 5th Ave./53rd St. Other location: 301 Park Ave., between E. 49th and E. 50th Sts., in Waldorf-Astoria, PHONE: 212/753-5595; Subway: 6 to 51st St.