Blue Opal from Chile
Opal is an extremely popular stone in its many phenomenal varieties. However, the market also offers countless types of nonphenomenal opal, whose selling point is their combination of color, pattern, and degree of transparency.
At the 22nd Street show, the author came across an enchanting ocean blue variety of opal from Chile, exhibited by Chilean Rocks and Minerals. The material on view was opaque to semitranslucent with an overall color banding and greasy luster, tumbled or cut as cabochons. The most prized pieces contained slightly greenish and vivid blue colors (figure 1, left). Less valuable polished stones showed alternating blue and solid white layers or the presence of green material and sometimes brown matrix in their bands. The cutters used the natural banding to their advantage, resulting in attractive colors and patterns that often evoke pleasant seascapes (figure 1, right). After cutting, the material is polymer impregnated to create a tough and durable finished product.
A selection of various grades of rough material was also available, displaying a dull conchoidal fracture with the dominant textural feature being the intricate interplay of blue and green, the two main colors (figure 2).
According to the dealer, this was the debut of cut Chilean blue opal at the Tucson gem shows. This type of opal is reportedly from the Atacama region, the only known occurrence to date in Chile, where a few mining operations are actively extracting the material. Presently known to the trade is a blue opal from Peru referred to as “Andean opal,” but that material is generally translucent and lower in saturation than the Chilean blue opal.
No detailed mineralogical analyses on the samples exhibited at 22nd Street have been reported. The blue color could be due to internal light scattering and dispersed chrysocolla, a mineral known for its vivid blue hue. Chrysocolla is commonly found in copper-rich deposits such as the porphyry copper deposits in the Atacama region. The analogy with Peruvian blue opal, which comes from a similar chrysocolla-rich geological setting, supports this hypothesis.
Given its vibrant color saturation, ability to take a high polish, and the desirable texture and alternating color patterns of the cut stones, this Chilean blue opal was a pleasant new addition to the gem offerings from Tucson 2024.
