There are many feldspar (pronounced FELLd-spar) gemstones; Sunstone, Andesine, Labradorite, Moonstone etc. etc..... 

But how are they different?  What is special about much raved Andesine?  Those were my original questions.  I was so curious that I wanted to clarify what are the feldspars and researched extensively for a while.  The answer was quite interesting.  Accidentally I have learned about the Andesine scandal in addition to the classification of the gem feldspars.  I want to share the infomation with you.

Actually the feldspars are the most common minerals.  They make up nearly 60% of the Earth's crust.  They are heavily used in the ceramics and glass industry.  They exist in granite along with quartz, and are used as building materials.  But gem quality feldspars with good color and clarity are much rarer. 

All the feldspars are aluminum silicate minerals.  Aluminum silicate forms the crystal frame of feldspars.  Potassium (K), sodium (Na), calcium (Ca) or barium (Ba) occupy the crystal cavities.  They are called potassium feldspar, sodium feldspar, calcium feldspar and barium feldspar respectively.  

But how are they related to the gem feldspars?  Here is the answer.

Potassium Feldspar

They are also called Orthoclase Feldspar.  Gemstones in this category are Noble Orthoclase (transparent colorless, pale yellow to golden yellow) and Adularia Moonstone (blue sheen on clear to milky background).  Microcline (colorless, milky white, pink to blue-green) and Amazonite (light green to blue-green) are also potassium feldspars, but their crystal structure is different from Noble Orthoclase and Adularia Moonstone. 

Sodium Feldspar and Calcium Feldspar - Here Comes the Andesine.

These two groups of feldspars form mixtures in various ratios.  They are called Plagioclase Feldspar.  It is divided into 6 distinct varieties depending on the ratios.

1) Albite (0-10% sodium feldspar (Na) and 90-100% calcium feldspar(Ca)):

Albite Moonstone (blue sheen on colorless to milky to yellowish background) is a gemstone in this category.

2) Oligoclase (10-30% Na and 70-90% Ca): 

Sunstone with metallic glitter also called aventurine feldspar.

Recently new varieties of sunstone were found in Tanzania. They are Confetti/Illusion Sunstone that has glittery hematite inclusions and pale blue Sunstone that looks like aquamarine with no hematite inclusions.

3) Andesine (30-50% Na and 50-70% Ca):

Andesine is valued for its clarity and color similar to Oregon Sunstone.    

• Beware of any gemstone sold by the name of Andesine.  There is no true Andesine on the market.  In 2008, the Jewelers Circular Keystone magazine reported in several articles that so-called Andesine were created from other cheaper feldspars with color-enhancement. 
• There is no andesine rough on the market.  This is odd, because feldspar is the most abundant mineral, and labradorite sunstone roughs (see below) are readily available.    

• As of July 08, Jewelry Television, which has been selling andesine as untreated, recently admitted a heat treatment on this stone (most likely a diffusion treatment with copper and iron), and offered a refund on returns for a short period.  The orignal color of these stones is yellow or colorless.  They may be common labradorite mined in India and Mexico, not Tibet, Mongolia or the Congo as first advertised.  A total hoax!

4) Labradorite (50-70% Na and 70-50% Ca):

Oregon Sunstone (pink, orange, red, green to blue, sometimes with copper schiller, sometimes pleochroic), Labradorite (metallic yellow, orange, green, butterfly blue flashes on gray to black background) and Spectrolite (from Finland, similar colors to Labradorite).  Clear champaign to golden sunstone is also called labradorite. 

• Oregon sunstone is never called andesine.  As of Jan. 09, Oregon US is the only source of gem quality clear and colorful Sunstone that is pink, orange, red, green and/or blue.  If you want a colorful faceted feldspar with high clarity, go for an Oregon Sunstone.

Labradorite from Canada showing labradorescence 

5) Bytownite (70-90% Na and 10-30% Ca): 

Bytownite (transparent yellow to slightly hazy yellow)

6) Anorthite (90-100% Na and 0-10% Ca):

Barium Feldspar

This is a very rare type of feldspar.  Gems in this group are Celsian and Hyalophane. 

Physical Properties of the Feldspars

Mohs Hardness: All within 6-6.5

Refractive Index: All within 1.5-1.6

Specific Gravity: All within 2.55-2.76

Conclusions

Now we know the feldspar gemstones are actually made up with different chemical components. 

Physical properties listed above can be determined with gemological instruments.  But, all the feldspars have very similar properties.  It is difficult to pin-point between the varieties, particularly when they have similar appearances like Oregon Sunstone and Andesine. 

• It turned out that "Andesines" supposed to be from Tibet, Mongolia or the Congo are the biggest scandal in the gemstone trade in the 21st century.  For details check www(dot)jckonline(dot)com and search for andesine. (Replace (dot) with .)

I often see Labradorite and Andesine side by side in a gem description.  They are in fact different feldspar varieties.  There seems to be huge confusion in the names of the gem feldspars. 

However, as physical propaties are all similar in the feldspar families, such a strict classification based on the chemical components may not be necessary unless you are studying mineralogy.     

This guide is updated from time to time, when more information becomes available.  If you learned something new and useful from this guide, please click "yes" below.  Thank you!!