Orange and red stones of every kind stood out at this year’s sensory-overloaded Tucson gem and mineral shows, but one major puzzle for gem lovers was an orange-red feldspar with an identity crisis.
At one exhibit, the dealer called the stone andesine from the Congo. His finest specimens were comparable in price to the best-quality tanzanite. Other companies also sold the stone as andesine, “Congo sunstone,” “red labradorite,” or occasionally the more factual “red feldspar.” And just to make things a little more confusing, the same material is also found in green.
The prices were literally all over the map. A Tucson vendor confessed that he and his colleagues snoop out their competitors’ prices. Within three days, they adjusted their pricing to be more competitive with each other.
What is red labradorite? Is it the same as andesine? Where does sunstone fit in?
The answer is complicated by the fact that it’s not a single stone from a single source. The gems sold under these descriptive names come from every corner of the globe, and so their chemical compositions vary slightly. The one thing they have in common is that they’re all feldspars.
Technically, the feldspar group has nine species, some of which look nothing like their relatives. All of the species we’re discussing fall under the heading of plagioclase feldspars, which includes a series of minerals between albite (a sodium aluminum silicate) and anorthite (a calcium aluminum silicate). See the chart on the next page for how it breaks down.
The gems sold as andesine are probably just that — a feldspar with the right blend of elements to put it in the andesine category. According to Laurent Sikirdji of Sikirdji Gemfrance, who was one of the first people to sell andesine from the Congo, “Other stones from the same location have been checked [and found to be] labradorite, near the andesine-labradorite boundary.” In other words, this andesine is so close to being labradorite that sometimes it crosses that chemical boundary.
Which brings us to perhaps the most confusing term of the bunch. The labradorite that most collectors are familiar with was originally found along the coast of Labrador around 1805; later it was found throughout Scandinavia and Newfoundland. This material is often called spectrolite. It shows brilliant flashes of green, blue, or red color, a phenomenon called labradorescence. The transparent to translucent crystals are usually cut with a flat surface in order to show off its iridescent effect — the look of aurora borealis.
But labradorite can also be colorless, yellow, or orange-red. This material is often found with some type of mineral inclusions — hematite, goethite, or copper, for example — that create a sparkling sheen. More commonly, however, it’s oligoclase feldspar that has those types of inclusions. In either case, the gemstone is called sunstone. So Oregon sunstone, for example, is gemologically a colorless to orange-red labradorite with copper “schiller” inclusions.
Most of the material being sold as “red labradorite” is probably just that — orange-red stones that fall into the chemical definition of labradorite. But given that these stones could come from China or various deposits in the United States, you can expect that the exact proportion of their chemical components could vary slightly from place to place.
Recently, I examined a colleague’s faceted green and red stones that seemed to shift their color, a characteristic seen in andalusite. The lab report concluded that his were bicolor stones.
I also received a bicolor labradorite in a recent appraisal parcel with red and green stones. A princess-cut red labradorite had a green color covering the bottom third of the pavilion.
The price and supply of these feldspars depend on what you’re talking about. Genuine andesine of facetable quality is very rare, and priced accordingly — up to $1,700 per carat for the best examples.
Fine-quality, facetable, transparent labradorites in a dramatic currant red or a rich pine green are sporadically available. What I’ve seen frequently has visible white streaks across the table facets. Nevertheless, the unusual gems are garnering loyal fans. The material that shows white streaking goes from $95 to $300 per carat; clean stones can be much higher. Much of the material on the market appears to have come from China and is available in large quantities, although some dealers are now selling red, non-schiller Oregon sunstone as red labradorite.
Fortunately, gem-quality red and green labradorite is still finding its market, and that presents some opportunistic buys. Will more be found? While we await the verdict, you may want to pick up the next clearly beautiful red or green feldspar you find.
Name that Pagioclase Feldspar
|Albite||90-100% albite||0-10% anorthite|
|Oligoclase||70-90% albite||10-30% anorthite|
|Andesine||50-70% albite||30-50% anorthite|
|Labradorite||30-50% albite||50-70% anorthite|
|Bytownite||10-30% albite||70-90% anorthite|
|Anorthite||0-10% albite||90-100% anorthite|