Blue Nile, James Allen, Lumera Diamonds, and several other online-only diamond sellers will ship stones (set or unset) to you. Also becoming popular are clicks-and-bricks relationships, in which an online retailer works with a local jewelry store. Customers can select several options online, then go to a store to compare the stones and finalize the sale. This is the approach used by New York-based Ritani, which will ship either unset stones or diamond rings to a local jeweler so you can evaluate a given gem in person before pulling the trigger.

You probably can save a lot by buying online. Checkbook’s undercover shoppers surveyed local and online stores for their prices for diamonds they would sell that met very narrow specifications. The results are shown here. For the most part, the best bargains were from online-only retailers, although some local dealers quoted prices that were competitive with the online stores.

Aside from price, another big advantage to shopping online is selection: Even for a narrowly specified type of diamond, you can choose from hundreds, perhaps even thousands, of stones. At first, having so many choices is overwhelming. But as you spend time sifting through all the grades and options, and using the websites’ fun-to-play-with filters, you’ll find that very small differences can save you a lot of cash.

On the other hand, since most online stores won’t ship you a dozen diamonds at a time for comparison (without charging you for all of them), you can’t easily examine stones side-by-side—a big disadvantage compared to shopping at the best local outfits.

If you do buy online, our advice about how to deal with local stores still applies: Limit your search to diamonds that have Gemological Institute of America (GIA) reports. Check return policies to make sure you can easily send back an item for a full refund with no questions asked. And take expensive purchases to an appraiser to make sure the seller sent you what was promised.

In addition, stick with sites that offer high-res magnified images of theirdiamonds. Other cool features of some sites include ring-building tools, which let you match stones to settings and bands. The fair trade-minded Brilliant Earth offers detailed photos, including a nifty feature that shows how big a stone appears next to a female hand.

Look for and read the fine print. Especially seek out terms that state the store can substitute a diamond if it can’t locate the exact one you bought. Most online stores sell from distributors’ inventories; if a store's database isn’t kept up to date, or it’s slow to request the diamond you bought, it might find yours was first sold to someone else. When that happens, rather than making a substitution the retailer should give you a refund and let you make a new selection, since the former arrangement opens the door to a bait-and-switch.