Gem Focus August 2020: Goldstone, Sunstone, Aventurescence; A Sparkling Enigma of Centuries
*Gem Focus & Market Pulse (Free Subscriber)GOLDSTONE, SUNSTONE,
AVENTURESCENCE;
A SPARKLING ENIGMA OF CENTURIES…
Gemstones with unusual optical effects are defined as phenomenal gems. Their unusual effect can designate a variety within a species and add value. i.e., star sapphire. Synthetic counterparts or simulants are often seen, especially when the natural counterpart is particularly valuable such as natural alexandrite and color change synthetic sapphire. Aventurescence is a phenomenon caused by light reflecting micro platelets in a gem and is easily recognizable from the glittering sheenlike effect. While green mica crystals in quartz creates aventurine quartz, hematite and ilmenite crystals create the same effect in sunstone feldspar.
Out of all the phenomena seen in gems, aventurescence, especially the yellow-brown type, has been the subject of a long-standing discussion. According to certain researchers, goldstone glass was produced centuries before the discovery of what we know as sunstone feldspar today. Goldstone is created by mixing microscopic copper platelets into molten glass. The sheen and earthy color of this material was ideal for Italian mosaics and more affordable jewelry, whereas much larger blocks were used to create ornamental objects such as snuff bottles, small bowls or carved figurines. A common story to its origin purports that the Italian glass producers discovered this particular effect by accident, but when this was is not clear. Accounts of the first production date vary from one source to another and are stated as from 11th century to 13th century. It is true that this rather attractive Murano glass with reddish brown speckles was a popular and expensive glass production in Italy for many centuries but it is hard to believe that sunstone feldspars were not known at least where they are mined in the old world, for example India.
Goldstone glass snuff bottle with aventurine cap.
Two Fairies. 19th Century Chinese object made of goldstone glass.
Courtesy of The Met Museum.
The other interesting fact about sunstone is that the name creates confusion even amongst gemologists. While historical records claim sunstone as the navigation aid to Vikings, recent studies point out that Vikings used calcite rhombs for its strong doubling for navigation. Thus, feldspar may not be the fabled “sunstone.” Correct gemological term for feldspars with aventurescence should be aventurine feldspar. Afterall, the sparkling effect created by metallic inclusions in various feldspar species change from one location to another. Feldspar is a solid solution series and it is almost impossible to come across a pure species of feldspar. In other words, even when we call a feldspar as orthoclase or labradorite, we are only referring to the highest percentage of its composition. Furthermore, not all sunstone feldspars are the same species. Oregon sunstone is labradorite and sunstone from Tamil Nadu, India is oligoclase. It is important to note that the value is affected by locality only in the fine to extra fine category. Oregon sunstone is the most popular and expensive of all as it displays both orange and green aventurescence, sometimes both in one gem creating a bi-color effect.
21ct bi-colored sunstone feldspar from Butte Mine, MT.
Courtesy of Mayer & Watt. Photo by Geoffrey Watt.
Separating natural material from the glass simulant is an easy task for a gemologist. Feldspars are doubly refractive minerals with two perfect cleavage planes. Therefore, with magnification, the inclusions would look regularly lined up with the crystal growth and cleavage planes would appear as fine lines. The same test would reveal the amorphous structure of goldstone glass with flow marks and irregular distribution of copper platelets.