The current featured stone:

 

Hackmanite

 
Refractive Index:  1.483-1.487 Crystal Structure: Cubic
 

 

 
Hardness:  5 1/2 - 6 Specific Gravity:  2.29 
 

 

 
Chemical Composition:  Na8Al6Si6O24Cl2
 
Occurrences:   Canada, Myanmar, Afghanistan

Phenomenal gemstones have always been of great interest to gem collectors; all of the intrinsic beauty and rarity of a gem with an added something special.  Hackmanite is a more recent addition to this list of stones that includes Alexandrite for its color change and Opal for its play of color. 

Chemically, Hackmanite is a variety of the mineral Sodalite, which is usually an opaque shade of violet blue that looks somewhat similar to Lapis.  Hackmanite is also rich in sulfur, which accounts for its strong UV fluorescence.  The real treat here however, is what happens after the florescent light source is removed.  Many Hackmanites, though not all, will undergo a temporary color change.  Stones that were colorless or light yellow to begin with emerge as deep purple beauties, sometimes tinged with a cranberry tone.  The color then fades away within a few minutes.  This phenomenon, illustrated in a piece of rough in the photo below, is known as tenebrescence.

The first faceted examples came out of the Mont St.-Hilaire deposit in the late 1980's and immediately became popular with collectors.  Stones have continued to trickle out of this locale and have subsequently been found in Myanmar (mainly cab grade) and, more recently a larger find of material in Afghanistan.  Unfortunately, most of these Afghan stones are colorless and do not experience the same color change as the Canadian stones.  

While not clearly not a mainstream jewelry gem, Hackmanite does make for a phenomenal (no pun intended) conversation piece and the material is durable enough to be set in a brooch or pendent.  The change is repeatable indefinitely and can sometimes be induced by strong sunlight in some samples.  Hackmanites are usually not treated in anyway and heating will irreversibly destroy the tenebrescence.