Gem artist Falk Burger collects the unusual. And nothing looks more unusual than these opals from Virgin Valley, Nev.

“This is ‘conk,’ which is an opal replacement of cavities in wood thought to be formed by a fungal disease,” says Burger, who specializes in cutting gems based on their natural features. “The term ‘conk’ is idiosyncratic but in common use among American opal fanciers. It’s the most sought-after—and hence the most expensive—of American opal varieties because of its beauty, stability, and durability.”

Burger notes that because the grain of the wood runs perpendicular to the surface of this stone, he will mount slices of conk onto a thin slice of black jade. As he points out, even though it is technically a doublet, “the opal itself is quite thick [4 mm].” Found by Keith Hodson and cut by Burger, the opal pictured here is an excellent example of wood replacement. “The wood was so rotten when the opal formed that only a few stringers permeate the opal.”

“In wood opal, even though the wood has been replaced by opal, it’s still fibrous and porous, and will not take a polish,” says Burger. “Therefore, all American wood opal has been treated with resins to allow the surface to be polished evenly. This treatment does not affect the opal, only the wood fibers.”

Burger can be reached in New Mexico at (505) 663-0710, e-mail: falkama@msn.com.