Following landmark articles on pink diamonds in 2002 and blues in 1998, the Summer issue of Gems & Gemology features an exhaustive look at the characteristics and grading of fancy-color yellow diamonds. The article details gemological data collected on more than 24,000 natural-color yellow diamonds examined at the GIA Laboratory in two calendar years, 1998 and 2003.

Yellow diamonds are among the most common of natural-color diamonds, but they are still a small fraction of the diamonds submitted to the laboratory. Thus, their valuations are much less understood than those of diamonds in the D-to-Z range. The goal of the study was to get a better understanding of yellow diamonds currently in the market, and also to determine possible trends in the types of such goods entering the industry during the past five years.  

The article also explains the procedures for grading and cutting fancy-color yellow diamonds, which are different from those employed for diamonds in the D-to-Z range. While yellow diamonds are typically classified as either type Ia or Ib, based on their nitrogen content, the authors have identified five subgroups characterized by spectroscopic features, color appearance and other factors.

Emeralds and Tourmaline

Zambia is considered the world's second-most important source of emeralds by value, behind Colombia. The second article in the Summer issue examines these emeralds and their associated deposits, which are located in the Ndola Rural Restricted Area of central Zambia near the Kafubu River. Most of the emeralds are mined from several open-pit concessions, and nearly all the rough is exported to India and Israel for cutting.

The article examines in detail the gemological, microscopic and chemical properties of emeralds from this area. When compared to emeralds from other localities, the combination of their physical properties, microscopic characteristics and chemical composition may be distinctive for the Zambian material.

The third major article in the Summer issue describes the revival of the Mt. Mica tourmaline deposit in Maine. Although the area has produced gem crystals since the 1820s, most tourmaline production there occurred between the 1890s and 1910, with only occasional finds thereafter. Since 2004, however, a new mining venture has yielded significant quantities of attractive yellowish green and greenish blue tourmaline. Some pink, green and bi- or tri-colored stones have also been faceted.

Summer Lab Notes

GIA's East Coast Laboratory recently encountered two fracture-filled diamonds that posed identification challenges. The first, a yellow 9.01 carat rectangular modified brilliant, was quickly identified as an irradiated diamond. However, while the fractures in this diamond were easily visible, some had an unusual texture and flow structure. A closer examination revealed the telltale flash effects indicative of fracture filling.

The second diamond, a 1.52 carat marquise, also showed obvious fractures not normally associated with a clarity-enhanced stone. A close examination with a microscope again revealed unusual patterns within the fractures and a weak flash effect. The Laboratory was unable to determine the exact reasons for these unusual-appearing filled fractures, but post-filling heat treatment or mishandling may have been responsible.

The East Coast Laboratory also examined an 8.38 carat round brilliant diamond with unusual laser drill holes. The diamond had large cavities that were unlike previously encountered natural etching. However, where the cavities reached the surface, their outlines did not have the rounded configurations usually seen with conventional laser-drilling processes. The cavities showed long grooves, some dark, that ran their length, which could have caused them to be mistaken for naturally occurring inclusions.

The West Coast Laboratory reported on a strand of what appeared to be "golden" South Sea cultured pearls that they received for identification. The Laboratory found evidence the strand had been dyed - with yellow on the string knots between the cultured pearls and color concentrations around the drill holes - but also discovered another interesting characteristic: Rather than the expected bead nuclei, an X-radiograph revealed that they were freshwater tissue-nucleated cultured pearls.

Gem News International

The Lake Baringo area of Kenya is a new source of ruby and pink sapphire. The deposit, of unknown size, has yielded some 350 stones, the largest of which weighed 3.71 and 2.80 carats. Preliminary indications suggest that this area has the potential to become a significant source of untreated ruby and sapphire.

Demantoid garnet from northern Pakistan made its first U.S. appearance at the 2004 Tucson gems shows. The small deposit is located in the Kaghan Valley near the country's peridot deposits. A report in the Summer issue provides a detailed gemological analysis of one matrix specimen and three faceted pieces.

A proprietary two-part (bleaching and stabilization) process is responsible for the attractive dark brown to light yellow-brown "chocolate pearls" that have appeared in the market recently. Only certain Tahitian cultured pearls can be used for this process, the distributor reports, adding that the treatment does not use dye or heat and the color is stable to conditions of normal wear.