GEM FOCUSAugust 2020

GEMGUIDE u GEM FOCUS - 1 - AUGUST 2020

Gemstones with unusual optical effects aredefined as phenomenal gems. Their unusualeffect can designate a variety within a

species and add value. i.e., star sapphire. Syntheticcounterparts or simulants are often seen, especiallywhen the natural counterpart is particularly valuablesuch as natural alexandrite and color change syn-thetic sapphire. Aventurescence is a phenomenoncaused by light reflecting micro platelets in a gemand is easily recognizable from the glittering sheen-like effect. While green mica crystals in quartz cre-ates aventurine quartz, hematite and ilmenite crys-tals create the same effect in sunstone feldspar.

Out of all the phenomena seen in gems, aventures-cence, especially the yellow-brown type, has beenthe subject of a long-standing discussion. Accordingto certain researchers, goldstone glass was pro-duced centuries before the discovery of what we

know as sunstone feldspar today. Goldstone is cre-ated by mixing microscopic copper platelets intomolten glass. The sheen and earthy color of thismaterial was ideal for Italian mosaics and moreaffordable jewelry, whereas much larger blockswere used to create ornamental objects such assnuff bottles, small bowls or carved figurines. Acommon story to its origin purports that the Italianglass producers discovered this particular effect byaccident, but when this was is not clear. Accountsof the first production date vary from one source toanother and are stated as from 11th century to 13thcentury. It is true that this rather attractive Muranoglass with reddish brown speckles was a popularand expensive glass production in Italy for manycenturies but it is hard to believe that sunstonefeldspars were not known at least where they aremined in the old world, for example India.

GOLDSTONE, SUNSTONE, AVENTURESCENCE; A SPARKLING ENIGMA OF CENTURIES...

Fracture filling in diamond displaying pink flash color. Photo by Gary Roskin.

Resin in emerald fracture displaying blue flash color.Courtesy of AGL New York.

August 2020

GEMGUIDE u GEM FOCUS - 2 - AUGUST 2020

GEM FOCUS

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The other interesting fact about sunstone is that thename creates confusion even amongst gemologists.While historical records claim sunstone as the naviga-tion aid to Vikings, recent studies point out thatVikings used calcite rhombs for its strong doublingfor navigation. Thus, feldspar may not be the fabled“sunstone.” Correct gemological term for feldsparswith aventurescence should be aventurine feldspar.Afterall, the sparkling effect created by metallic inclu-sions in various feldspar species change from onelocation to another. Feldspar is a solid solution seriesand it is almost impossible to come across a purespecies of feldspar. In other words, even when wecall a feldspar as orthoclase or labradorite, we areonly referring to the highest percentage of its compo-sition. Furthermore, not all sunstone feldspars arethe same species. Oregon sunstone is labradoriteand sunstone from Tamil Nadu, India is oligoclase. Itis important to note that the value is affected by local-ity only in the fine to extra fine category. Oregon sun-stone is the most popular and expensive of all as itdisplays both orange and green aventurescence,sometimes both in one gem creating a bi-color effect.

Separating natural material from the glass simulant isan easy task for a gemologist. Feldspars are doublyrefractive minerals with two perfect cleavage planes.Therefore, with magnification, the inclusions wouldlook regularly lined up with the crystal growth andcleavage planes would appear as fine lines. The sametest would reveal the amorphous structure of gold-stone glass with flow marks and irregular distributionof copper platelets. u

21ct bi-colored sunstone feldspar from Butte Mine, MT. Courtesy of Mayer & Watt. Photo by Geoffrey Watt.