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This month's stone profile:

 

Color Change Garnet

 
Refractive Index:  1.59 - 1.65 Crystal Structure: Cubic 
 

 

 
Hardness:  7 - 7.5 Specific Gravity:  3.85
 

 

 
Chemical Composition: Mg3[Mn3]Al2Si3O12 plus ~1% V2O3  Occurrences:   Madagascar, Tanzania

As a variety, color change garnet is not a new discovery.  African stones have been in the market for quite sometime, including some smaller stones with strong, near Alexandrite like changes.  Recently though, a new source in Madagascar has produced examples which are significant for their colors which seem to be induced by Vanadium.

A fair number of gemology texts still praise the Garnet group as being found in nearly every color, except blue.  The last few years has done much to eliminate this final caveat, as the the daylight color of many of these pyrope-spessartite varieties comes very close to a dominant blue.  Moreover, the transition from very saturated purple in incandescent light makes for a strong, noticeable color change.  The size range for this material is heavily restricted to stones under 1 carat.  1-2 carat stones are available, though above this size availability becomes greatly limited.  We have yet to see any stones above 5 carats, though some reports quote the largest examples at around 9 carats (see Gems & Gemology, Winter 1999, pp. 196-201).

As a source for new gemological discoveries, it is hard to ignore the near dominance of Madagascar.  In last couple of years we have seen a host of new materials from this island nation, including Londonite and the new Beryl analog, Pezzotaite, which made quite an entrance to the gem world two years ago. The source of the color-change Garnets is near Bekily in the south of the country though, not surprisingly, there is not a great deal of information available to unconnected sources.  Several suppliers at Tucson this year mentioned the diminishing availability of these stones, consequently we may have to make due with limited quantities of this material for quite awhile.