The same thing, but different
An effect called ‘colour shift’ occurs in untreated stones, but it’s not the same thing. “It is important to distinguish colour-change from what is sometimes called colour shift,” explains Bangkok wholesaler, Arnold Silverberg, owner of AJS Gems. “Many gemstones show some degree of colour shift within their hue family under different lighting conditions. Blue sapphire, for example, looks bluer under natural light than [it does] under incandescent lighting because natural light is richer in blue wavelengths. The colour of ruby, on the other hand, is enhanced by incandescent lighting, which is weighted more heavily to frequencies in the red and yellow part of the spectrum.”
Study plenty, then cut
Gem cutters must become expert in each colour-change species to achieve its maximum potential. Vancouver faceter Lisa Elser tells Jewellery Business, “There are a few challenges. Orientation is critical to get the best possible change, plus two good colours from the rough. The material is usually quite expensive, so I must balance colour-change with the stone’s finished carat size. Colour-change gems, in particular red/blue or red/green garnet and Umba sapphires, typically have fine, silky inclusions that can make the finished gem look ‘sleepy’ if I’m not careful.”
Cutting colour-changing stones can become more complicated when the country of origin requires its gems be cut prior to export. “I prefer to cut from my own rough,” Elser shares. “In Sri Lanka, it was impossible to acquire high-quality rough for export. Most of the poorly cut stones would have made good recuts, but were priced far too high to be workable. They may be able to sell a poorly cut stone, but I’d lose too much on a recut.”
Ontario custom cutter Rudi Wobito is known for his artful zultanite creations, which he began cutting in 2004. “In the case of zultanite, the crystals are all highly included and yield only two to two and a half per cent from rough,” he explains. “To get a deeper impression of colour, you need to cut stones with smaller facets and a thicker girdle. [With zultanite’s crystal structure,] you get the strongest colour by cutting at right angles to cleavage planes, yet it’s almost impossible to cut that way. The next best solution is to go one to two degrees off the cleavage plane.” Wobito stresses that with zultanite and other colour-change gems, it’s always a compromise between weight retention and optical effect.
In addition to the extent of colour-change expressed as a percentage of the entire stone, the attractiveness of the gem’s two distinct colours is the most important component regarding value. These colour changes are infinitely repeatable, giving the wearer and admirer a visual treat each time the stone moves into a different lighting source.
Since value is dependent on stone quality, dealers and cutters must trek the perilous back roads of the world, sleuthing for the good stuff. Gem cutter Roger Dery does just that to hand-pick stones worth cutting. “The bounty really of garnet is coming out of southern Kenya and northern Tanzania,” he explains. “The preference for most buyers is the Kenyan colour-change garnet. It is more revered, as its qualities are more alexandrite-like.”
