Black Chalcedony

Okay, if you want to call it black onyx, go ahead. Everybody else in the industry does. But by definition, onyx should have straight parallel stripes of black and white … and these gems don’t.

The stone is chalcedony, a cryptocrystalline—or microcrystalline—quartz. Examples of chalcedonies of just one color are chrysoprase, a yellowish-green; carnelian, a brownish-orange; and chrysocolla, a greenish-blue. Pure black chalcedony is rare. Most chalcedony is multicolored and banded and is called agate. Other multicolored chalcedonies include bloodstone, the March birthstone, which shows red splotches on a green background.

What is onyx? There are two traditional onyxes: black onyx (black and white parallel-striped chalcedony) and sardonyx (orangey-brown and white striped chalcedony). When those bands are wavy, curved, or irregular, the chalcedony is called agate.

Because chalcedonies are porous, they are easily dyed, which means you will encounter all colors of onyxes and agates. Such chalcedonies are lumped together as simply “dyed agate” or “dyed onyx,” for either wavy- or straight-banded colored chalcedony. Gem engravers and carvers often create unusual colors of parallel-banded chalcedonies for use in special-design cameos. When the gem is completely black, it’s supposed to be called black chalcedony—not onyx.

Most black chalcedony is enhanced, so technically it should be called “dyed” or “enhanced black chalcedony.” Of course, if you knew that the original material was an onyx, and now was dyed completely black, you could call it “dyed black onyx” … but generally it’s simply called black onyx.

Enhancements. Since we’re talking about an enhanced stone, you should know that there are a number of treatments for chalcedony, some permanent, some not. The more traditional organic dyeing of gem materials is still a common treatment for the inexpensive material you will find in the market, but the colors may fade if left in direct sunlight. The enhancement process most gem artists prefer is one that probably dates back to the days of the Roman Empire. The stones are soaked in a sugar bath (probably honey back then) for days or even weeks, then placed in a hot bath of sulfuric acid to carbonize the sugar. This is a penetrating and permanent process.

However, portions of chalcedony that may not be porous—such as the white bands in cameo material—will not accept dyes or sugar treatment.

Heat also may be used to change pale colors to a milky white. This is frequently done before dyeing in preparation for creating cameo and intaglio material.

Qualities. There is not much difference from one gem to the next. You either like it, or you don’t. It will either be porous enough to accept the enhancement, or it won’t. This consistency is one reason gem artists prefer chalcedony to many other gem materials.

One really never knows what will be inside the nodules of chalcedony when they are sliced open and slabbed before processing. There may be a carpet of tiny crystals, called druse, lining a cavity inside the nodule. Gem artists favor such uncommon features and like to incorporate them into finished pieces.

The druse crystals are single transparent crystals that are not porous; therefore, they take on the color of the chalcedony layer directly below. Many times, that layer is a dense chalcedony that is also nonporous and typically gray in color, although it can be white.

Most chalcedony seen in the United States comes from South America. Brazil is the major source, but it’s said that the best cameo material, black and white, comes from Uruguay.

Pricing. Much of the pricing structure for black chalcedony is based on the quality of the design of the gem. Standard tablets for men’s rings are sold by the measurement for approximately $1 to $15 per piece. The price of a carving by a gem artist such as Steve Walters, Ramona, Calif.; Glenn Lehrer, Larkspur, Calif.; or Dieter Lorenz, Idar-Oberstein, depends on the artist’s conception of its value. “It’s like a blank canvas,” says Walters. “We charge for the labor, the wear and tear on our tools, and the years of experience to do something nice to the gem.”

Care and cleaning. Ultrasonic cleaning shouldn’t be a problem for chalcedonies that have been permanently enhanced. Polished black onyx scratches fairly easily and the scratches are noticeable, which is why most carvings are matte-finished. Druse is relatively easily abraded and chipped, so extra care should be taken to protect the tiny crystals.

Bench repair and setting. Prong setting, bezel setting, or combinations of the two are appropriate for this durable gem material. With a hardness of 7 and fairly good toughness, it is not easily damaged at the bench. However, as noted above, polished black onyx should be protected, even from the bottom of the ultrasonic cleaner. Also, black onyx is heat sensitive, so remove the stone before retipping or sizing.

Recommended reading. For more information, see:

Gemstone Enhancement, by Kurt Nassau, Butterworths, 1984.

Special thanks to Steve Walters, Ramona, Calif., and Don Olson, Bonsall, Calif.