September’s birthstone is perhaps most known for the extraordinary sapphire ring worn by Kate Middleton, originally belonging to Princess Diana. Taking a cue from the British royals, celebrities like Penelope Cruz have fueled the colored gemstone engagement ring trend, with sapphire leading the way.

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Kashmir sapphire ring from Tiffany’s.

Like its sister gemstone ruby, sapphire is a variety of the mineral corundum. Corundum in its purest form is colorless. These colorless sapphires are popular imitations for diamonds, and have made a comeback as accent stones in recent years. Sometimes when trace elements are present in corundum, color happens. The deep blue hue we most often associate with these gemstonnes results from a combination of the elements titanium and iron. Some combinations of elements can result in colors including purple, green, yellow, orange, pink, and black – the so-called “fancy sapphires.” (A ruby is a ruby and not a sapphire, when the mineral corundum contains enough traces of the element chromium to lend the gem a medium to medium dark red tone.)

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Multicolor briolette sapphires interspersed on strands of keshi pearls. Image courtesy of Pearl Exporting Company.

Known as one of the “Big Three” of jewelry gemstones (alongside ruby and emerald), sapphire is a popular and affordable choice. When shopping for a this gemstone, the quality of the cut makes a tremendous difference—a well-cut sapphire will maximize the color.

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10.64 carats of round sapphires accented with 7.47 carat of brilliant round diamonds set in 18K white gold. Image courtesy of Omi Prive.

The most common cuts are cushion and oval shapes, as well as emerald, princess and trilliant cuts in stones under a carat. When visiting a jeweler, compare the various shades of sapphire side by side to understand the range of qualities available. Most sapphires you’ll see in jewelry stores are treated by heat or lattice diffusion to alter their color. Lattice diffusion can enhance or change the color, but lattice diffused sapphires are considered less valuable than heat-treated ones. A GIA lab report indicates any detectable treatments and confirms if the sapphire you are considering is natural or synthetic.

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Assorted colored sapphires from the Dr. Edward J. Gubelin Collection.

Sapphire is one of the most durable gemstones—it has a hardness rating of 9 on the Mohs scale—which makes it relatively easy to care for. Cleaning this gemstone regularly with just a little mild dish soap, a toothbrush and water is enough to maintain its brilliance.

What is your favorite celebrity sapphire jewelry piece?