Hauyne, also sometimes called hauyn or hauynite is a rare tectosilicate mineral. It is a highly complex mineral and has the chemical formula (Na,Ca)4-8Al6Si6(O,S)24(SO4,Cl)1-2. 
Hauyne is a rare mineral and typically occurs as minute crystals under 5mm across. It is best known for its striking blue color - one of the most intense of all blue gemstones.
Hauyne is a member of the feldspathoid mineral group. Feldspathoids are closely related to feldspars, but are low in silica content. Feldspathoids are tectosilicates and the feldspathoid group also includes sodalite, lazurite, nepheline, leucite, analcime, cancrinite and nosean.  Hauyne is a component of lapis lazuli, and imparts its blue color to that stone. Hauyne is also commonly found in association with several other minerals including andradite, apatite, sanidine and titanian.
Hauyne was named after the famous French mineralogist / crystallographer René-Just Haüy (1743-1822), who discovered it in 1807 on Mount Somma in Italy.  Hauyne has now been found in several other locations including Canada, France, Italy, Namibia and the USA , however the best gem material is said to be that which comes from Nickenich, Germany. 
Despite its rarity and desirability (on account of its rich sapphire-blue color), hauyne can be seen for sale online relatively inexpensively - more often in rough form than faceted. However, not much material is available for sale at any given time, and most of the stones seen are minute - under 0.5 carat in size!
The best hauyne is that with rich, strikingly intense "electric blue" color, yet which has internal clarity, is not too dark and is eye-clean. One online auction currently running (March 2011) lists seven pieces of hauyne rough with a total of 0.82 carats of fairly good color, priced at $60; a price of $73 per carat. Other stones of lesser clarity and/or paler color can of course be seen at lesser prices - but almost all of what is seen is very small. Larger stones i.e. over 1 carat are extremely rare and much more expensive, if they can be found at all. The largest facetable hauyn ever found is said to have been a rough of 5 carats.  Even the hauyne example listed as an 'Important Gem' in the Smithsonian's National Gem Collection is only 0.32 ct. 
Hauyne has a hardness of 5-6 on the Mohs scale, and usually forms twinned crystals.  It has a refractive index of 1.494 - 1.509. 
In addition to the striking blue color for which it is famed, hauyne has also been stated to occur in white, gray, yellow, green, pink , brown, black or red  - although these statements are evidently controversial as one well known gemstone information source is skeptical that such stones exist.  The other colors of hauyne are at the least, very rare indeed.
Small faceted hauyne gemstones 1-2mm in size.
Photo by Ra'ike; image lic. under CC-BY-SA-3.0
(From Eifel, Germany. Weight: 1.96 carats total for all pieces in the image)
Image © supplied by Woodmansee* Gems
Hauyne - Sources Referenced:
 "The National Gem Collection" - Jeffrey E. Post (Abrams, 1997)
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