What necklace is right with a basic T-shirt, and what styles work best for formal wear? Here are three classic styles that are versatile enough to wear with anything.   Diamond Solitaire Necklace: The understated simplicity of the diamond solitaire necklace has made it a fashion classic. It is truly the little black dress of jewelry, right for a variety of occasions from day to evening. Depending on taste and budget, there is a wide range of styles available, including modern bezel settings on delicate yellow or white gold chains.   Cultured Pearls: Quietly elegant, a strand of cultured pearls goes with everything from a classic sweater set to a wedding dress. Pearls complement other types of jewelry with ease, from colored gems and diamonds, to gold, silver and platinum. A 16-inch strand of white Akoya 7mm pearls is a timeless necklace style, not too long and not too short.   Pendants: can be elegant or casual depending how you wear them. Feminine styles prevail in popular motifs - a heart, star, ring or simple cross can be dotted with tiny colored gems dangling from beaded or link chains that are 18 inches or longer.