What distinguishes the woman looking great in costume diamonds from the one you want to save from her own bad taste? It typically comes down to two factors: what she's wearing, and how she's wearing it. What she's wearing is perhaps the easier set of characteristics to define.
It's fairly straightforward: finishes and gems.
Finishes are the colored coatings that turn the chain, setting, French hooks, clasps and all other metal components into, typically, silver or gold tones. Of course, some high-end costume pieces actually are silver or gold. Those are the relatively expensive ones, and are sometimes referred to as "bridge" jewelry. They can run into the thousands of dollars and often sport very real-looking fake stones.
More often, we're talking about the cheaper stuff -- "fashion" jewelry, which you can typically pick up for under a $100 (sometimes way under). In those pieces, the metals usually aren't precious, and finish quality can vary dramatically. What you want to look out for is the cheap, flimsy coating you can scratch off with a finger nail. That stuff looks cheap, acts cheap and is cheap.
Typically, the nicer pieces will be electroplated in real silver or gold. That's when manufacturers chemically bond a thin film of precious metal to a base of non-precious metal. It's a higher-quality finish that looks good and stands up to normal wear.
That's the background, and it's a crucial component in wearing cheaper jewelry without looking cheap. The other major factor is the centerpiece -- the gem.
Expensive-looking costume gems can be made of a variety of materials and take countless forms. They may closely resemble diamonds, rubies, pearls or emeralds, or they may create a look all their own. It's perhaps easier to define quality costume gems by what they're lacking, namely: paint, foil backing and lightweight plastic. And they're not glued on -- they're prong-set or hand-tied.
With an eye out for materials and mountings, classy costume jewelry is pretty easy to spot. Wearing it with class, on the other hand, can be a bit more complicated.