If you are purchasing the ring for your partner unbeknownst to said partner, you will have to do a bit of sleuthing. First you will need to find out if your partner wears any other jewelry, and if so, what kind of style does the jewelry appear to fall into-- does it look antique or antique reproduction? Is it primarily gold or silver? Or a bit of both? Modern? Does she or he buy jewelry from museum catalogs (The Metrapolitan Museum of Art, for example)? If so, you'll want to find a jeweler whose pieces have the same antique feel, like Mallory Marks who makes magical, delicate pieces that might as well have been worn by Marie Antoinette or Catherine de Medici-- these can be found at Barneys New York (http://www.barneys.com) among other places. Henri Ramon Designs also plays around with the idea of the magical antique, often marrying different metals with luminous stones. A recent Henri Ramon ring consisted of a platinum band with a rectangular faceted Tanzanite stone (more purple than sapphire but just as precious) held in a gold cage composed of two miniature and indescribably delicate angels clasping each other's wings.
Does your partner hardly wear any jewelry at all? If so you'll want to go with something incredibly simple and elegant-- a small stone in an even smaller setting, in a color of metal that you think would go with the clothes your partner wears. Masters of simple, unelaborate settings include the jewelry artist Dale Novick, but you can easily find a simple design by conferring with any designer. Does your partner like to dress extravagantly in rich silks and velvets, sparkling from head to toe? A pave diamond ring may be just the thing, from Codiam Inc. a firm that specializes in modern pave settings, or from an antique dealer that specializes in late nineteenth century, early 20th century antique jewelry. If your partner's personal dressing and jewelry style is considerably more modern, and always follows the sharpest, latest trend then you might want to choose something that mixes platinum with gold, such as a design from Henry Designs New York, or a pure platinum from L. Pavorsky of Philadelphia.
If you still can't figure out ot decide what setting will complement your partner's dressing style, here is one more place to look: decorating! How does your partner decorate her or his room/house/apartment? With thoroughly modern, sleek, minimalist furniture and Japanese vases? (go with something minimalist and simple-- platinum, or else something inspired by nature like the work of Laura Cardillo, or Russell Trusso). With a funky "shabby chic" mix of flea market "finds", European prints and vintage appliances? (Try something modeled after an Art Deco design with square or rectangular cut diamonds, or another funky vintage-like design, diamonds set in Bakelite or resin, for instance!) With a perfect Laura Ashley country-house mix of chintz and refurbished antiques? Best to go with something classic that looks both 20th century and timeless. If your partner's house is draped with magical fabrics, constantly lit with baroque candlesticks and dripping with rose petals (in other words, the setting for a romance novel or a fairy tale) then try something imaginative and elaborate, full of Art Nouveau style swirls and curlicues, in the style of LaLique, or from the still up and running Masriera collection. With artifacts from around the world, Eastern European rugs and kilims, Indian spice boxes and lots of orchids? Try a culturally inspired design that mimics pieces found in museums, like the ancient Greek and Byzantine style of the work of Maua Neimanis, which often involves "granulation", or tiny spherical beads of precious metal placed closely together to form a repeated patterned design. Granulation is done completely by hand, so these pieces can be costly, but always receive compliments on their unique, historically informed design.
No matter what you do, buying "on the sly" always carries its risks. Be sure you understand your jeweler's return policy, in case you happen to misread your partner's preferred style.