(From Tanzania. Weight: 0.76 carat)
Image © supplied by Woodmansee* Gems
The plagioclase feldspars are what's known to mineralogists as a solid solution series. This means that they are effectively a "mixture" of two separate minerals, known as "end members"; in this case albite (NaAlSi3O8) and anorthite (CaAl2Si2O8), with the proportions of the two components determining the name and properties of the resulting mineral. Several minerals with different proportions of the two end members have been differentiated and named - and oligoclase is made up of 70-90% albite and 10-30% anorthite. 
Oligoclase was first recognized and named in the 1820s. It has a hardness of 6 to 6.5 (Mohs) - and as can be seen in the top photo of Oligoclase from Brazil, clear and pale-colored / colorless oligoclase has been used as a gemstone; although only a small proprtion of the oligoclase found is of gem / translucent quality. Perfectly colorless oligoclase has also been found in North Carolina, USA. 
Another variety of oligoclase may more commonly be seen in the world of gems, and that is sunstone, also known as aventurine feldspar (not to be confused with the aventurine which is a type of chalcedony! Sunstone is typically reddish with sparkles of metallic golden color.
Some examples of oligoclase of been found to display fluorescence. 
Oligoclase has been found in numerous locations worldwide, including Czech Republic, France, Norway, Germany, Italy, Russia, China, Australia, Hungary and in many places in the USA.  As it is a common mineral, gems can be had for reasonable prices - and to give you some idea of the price range, a quick search online (2010) showed a 0.84ct colorless faceted oligoclase had recently sold on ebay for $12. I also saw an eye-clean round cut blue oligoclase of a lovely aqua color, 2 carat, priced at $122, and a rectangular cut 3.8 carat of a similar color, with inclusions, priced at $135.
Oligoclase Gemstones from Brazil.
Photo by Eurico Zimbres; released under CC-BY-SA-3.0 license
Oligoclase - Sources Referenced:
 http://www.geology.neab.net/minerals/oligocla.htm  http://www.mindat.org/min-2976.html
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