Why Some Gems Are Cut as Cabochons

A cabochon is a gemstone shaped and polished with a smooth, rounded top and usually a flat base. Gems are cut en cabochon when a dome displays their color, pattern, opacity or optical phenomena better than facets would. Many opals, turquoise, onyx, moonstones and star sapphires are traditionally finished this way.

In the gemstone trade, cabochon is often shortened to “cab.” Cabochons may be made in many outlines, but oval is by far the most common. You can also browse cabochon cut stones for sale to see how widely the style is used across different gem materials.

What Makes a Gemstone a Cabochon?

Although lapidaries commonly say a cabochon is “cut,” it is more precisely shaped and polished rather than faceted. A cabochon normally has a convex, domed crown and a flat or slightly curved back. The surface is continuous and smooth, without the flat faces used on faceted gemstones.

This simple form is not a lesser style of gem cutting. For many materials, it is the best way to reveal the gem’s visual character. A cabochon emphasizes body color, surface pattern, translucency and special light effects, while a faceted cut is designed mainly to return light as brilliance and sparkle.

Cabochons Came Before Faceted Gems

Before the development of faceting, gemstones were generally polished as cabochons, beads or carvings. Faceted gemstones began appearing in European jewelry toward the end of the 13th century and the beginning of the 14th century. Later, the horizontally turning cutting wheel, introduced in the late 1400s, made it easier to create and repeat more complex geometric facet patterns.

As lapidary skill advanced during the Renaissance, gem cutters increasingly focused on controlling light inside transparent stones. Even so, cabochons remained important because many gemstones are not improved by faceting. Some look better, or reveal essential optical effects, only when polished with a rounded dome.

Why Some Gemstones Are Better as Cabochons

Cabochons Reveal Optical Phenomena

Some gemstones are cut as cabochons because their most valued feature depends on a curved surface. Optical phenomena such as asterism, chatoyancy, adularescence and opal color effects are usually best seen on a polished dome.

• Asterism: the star effect seen in gems such as star sapphire.
• Chatoyancy: the cat’s eye effect, especially well known in chrysoberyl cat’s eye.
• Adularescence: the soft, floating glow associated with moonstone.
• Opal color effects: the shifting color display that makes many opal cabochons so distinctive.

For cat’s eye gems, the cabochon dome allows the bright line of light to appear across the stone. This is why cat’s eye cabochons are cut and oriented very differently from transparent faceted gems.

Opaque Gems Usually Look Better Polished Than Faceted

Most opaque gemstones are cut en cabochon rather than faceted. Since light cannot pass through them in the same way it does through transparent material, facets do not create the same brilliance. A smooth polish is usually more effective for showing even color, matrix patterns, banding or surface figure. Turquoise and onyx are classic examples.

Cabochons Make Good Use of Attractive but Less Transparent Material

Some gem species can be either faceted or cabochon cut, depending on the quality and transparency of the rough. Sapphire, ruby and garnet are common examples. If the material has pleasing color but is too included, translucent or unevenly transparent for an attractive faceted stone, it may still make an excellent cabochon.

Softer Gemstones Can Be More Practical as Cabochons

It is also common to cut softer stones as cabochons. Gems with a Mohs hardness below 7 can be scratched by quartz particles in everyday dust and grit. Fine scratches are generally less obvious on a rounded, polished cabochon than on a faceted stone, where abrasions can show along facet edges and flat surfaces.

Cabochon or Faceted: The Main Difference

The choice is therefore not simply a matter of fashion. A gem cutter chooses a cabochon when the shape suits the material. For many colored gemstones, the smooth dome of a cabochon is the most effective way to show the gem’s natural beauty.