GARNETS presented Oct. 3-2007 by Andre Mongeon and Stephanie Martin

6 common varieties form two related groups:

Aluminum garnets: (pyralspites) pyrope, almandine, spessartine
Calcium garnets: (ugrandites) uvarovite, grossular, andradite

Type				Usual colour	Usual environment
-----------------------------   --------------  ---------------------------

Pyrope  Mg3Al2Si3O12      	Red		peridotite or serpentines or
					 	alluvial deposits

Almandine Fe3Al2Si3O12 		Purple-red  	schist

Spessartine  Mn3Al2Si3O12	Orange/brown	pegmatites, sometimes
						metamorphic rocks

Uvarovite Ca3Cr2Si3O12		GREEN		usually on chromite

Grossular Ca3Al2Si3O12		Many colours	metamorphosed limestones

Andradite Ca3Fe2Si3O12		Black, green 	Igneous and metamorphic
				or brown (not red)  environments

Often garnets fully mix within their group and sometimes partially 
between the 2 groups.

Andradite-cored grossulars of Coahuila, Mexico. 

These colourful garnets are not gem grade but their colour can be
striking.  The manganese tinted grossulars are sometimes called
Rosolite.  Other colours range from grey to tan to light green, all of
which are virtually opaque.

Iron was present when the Andradite core was formed; Aluminum replaced
Iron when outer shell of grossular was formed.  However, the
Iron/Aluminum levels fluctuated during their formation causing
'banding' of the two types of garnet.   Without this fluctuation, the
garnet would have likely been an even mix or a simple pure andradite
coated by a pure grossular shell.  Some cycle seems to be responsible
for the banding.

Rainbow garnets of Tenkawa, Japan.

These andradite garnets have a Labradorescent play of colours.  The
phenomenon is likely caused by a banding of slightly different garnets
as they formed.  Each band of garnet has a slightly different
refractive index than adjacent layers do. This variance causes light to
split into a 'rainbow'.  This is phenomenon is similar to that of
Labradorite, where different layers of feldspar cause a play of
colours.

References

Pough, Frederick H.  A Field Guide to Rocks and Minerals 5th edition.  
New York: Houghton Mifflin Company, 1995.

Alan Guisewite's page on Garnet
http://www-2.cs.cmu.edu/~adg/adg-pgpimages.html

Mindat page on Garnet Group
http://www.mindat.org/min-1651.html

Red Grossular from the Sierra de Cruces, Coahuila, Mexico (from 
Mineralogical Record Nov/Dec 2003 in Array provided free by LookSmart 
Find Articles).
http://findarticles.com/p/articles/mi_qa3672/is_200311/ai_n9339643/pg_1

Rainbow Garnet from Tenkawa Village in Nara
http://www.gaaj-zenhokyo.co.jp/researchroom/2005/2005_12b-01e.html