That's its trade name, of course. Known to marine biologists as melithaea ochracea (say that five times fast!), sponge coral really has nothing to do with sponges. It just looks like a sponge.
Gem corals are hard, underwater, organically produced tree-like structures usually found attached to sea floors. These underwater trees are the byproducts of small cylindrical, tube-like colonizing sea creatures called polyps. The holes in the sponge coral are the homes of these individual polyps. Unlike most other gem corals, the sponge coral polyp homes rarely grow shut, and these open vacant homes are what have given the sponge coral its name.
Locality. Found mainly in the South China Sea, from Taiwan down to Indonesia, the sponge coral has a distinct fan-shaped appearance underwater. Many ornamental sponge corals have been used—and simulated—for decorative use in aquariums. Until recently, sponge coral was not used for jewelry: They weren't considered jewelry-quality coral because they had too many holes.
Importation of wildlife into the United States, whether alive or dead, requires a $50/year import-export license/federal fish and wildlife permit from the U.S. Department of the Interior's Fish and Wildlife Service. Red corals used in jewelry do not require any other permits, as they are not on any endangered species list. This includes the melithaea species of sponge coral. All entries of coral must be cleared by the U.S. Fish and Wildlife Service prior to release by the U.S. Customs and Border Protection Service.
Quality. The color of fine-quality sponge coral is a strong orangey-red. It is typically found variegated with fine yellow or orange trails, with the lesser quality showing larger variegated areas as well as a secondary brownish tint. It is difficult to say whether the nonvariegated or slightly variegated pieces are more desired, as each is unique in its own way. As for color, however, the closer to pure red corals are, the more sought after they are. There also are blue sponge corals, the majority of which are accented by a secondary grayish hue. Sponge-like coral of a pure, dark, saturated blue color is heliopora coerulea, and free trade is forbidden.
Enhancement. According to Peter Rohm, coral and gemstone manufacturer in Austria, because the structure is full of holes, 95% of all sponge coral is stabilized. "You can recognize this easily, as the holes are filled when stabilized," says Rohm. While most corals can show a high polish, if you look at a sponge coral and see a smooth surface, that means it has been filled with a resin or polymer. Using a microscope or loupe, one can usually spot individual filled holes fairly quickly.
Along with being filled, some material also is dyed, and a small amount of sponge coral has reportedly been "pressed"—crushed up, mixed with epoxy, and formed into desired shapes.
There is some natural unenhanced sponge coral in the market, but according to Rohm, it is much more expensive: "Only very good rough material can be kept natural." Oils may be used on corals that have not been filled.
Price and availability. "There is quite a lot [of sponge coral] on the market," says Rohm. Currently, it is relatively inexpensive, with sufficient material available for the near future. It has been popular among jewelry designers because of its price, because it's easy to carve or shape, and because of its color. As Rohm notes, "The color red has been in fashion for almost three years now."
Care and cleaning. Coral is soft, approximately a 3.5 on the Mohs hardness scale. While this makes it easy to work with, it also makes it easy to damage.
As for wearability, according to Rohm, coral in general should not be we worn directly on the skin, and may be washed regularly with light soapy water.
Special thanks to Peter Rohm, Rohm GmbH & Co Kg, Linz, Austria ([43 732] 777-8870, www.rohm.at.), who provided all sponge corals.