Delaware mineralogical society

A Delaware 501(c)(3) non-profit earth-science educational organization


"Sillimanite: Delaware's Official State Mineral and Benjamin Silliman, A Tradition of Firsts"
by Ken Casey

Aluminum Silicate

What's In A Name?
On a mission
Chemistry & Science
Delaware Sillimanite Geology
Brandywine Springs Park
Two Museums of Note
30th Anniversary of Sillimanite
Members' Gallery
Article Contributors
Photo & Graphics Credits
Suggested Reading
Invitation to Members
Past Minerals of the Month

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        Delaware State Flag

Image courtesy of Marchex, Inc.
©2007, World Flag Database

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Hurray! It's Official!...


...Yes, Sillimanite is the Official State Mineral of Delaware--for over 30 years now!

(Top, left): Sillimanite in creekbed at Brandywine Springs Park
Photo by Ken Casey ©2006

(Top, right): Fibrolite/Sillimanite sample from Fran Poniecki
Photo by Ken Casey ©2006


     This month, we explore in our own backyard: Sillimanite.

       "The official State Mineral of Delaware" by vote of our Legislature, this native mineral
had initially captured the attention of our club in 1977.  It's story lies herein. 
Let's go!



     Welcome to this year's first installment Mineral-of-the-Month! 

     This January, we will uncover what makes Sillimanite so popular to our residents, and some
items on it's formation in Delaware, and in general.

     We seem to have formed a fondness for this simple silicate.  I'll share with you why we like
it so much.  Enjoy!


What’s in a name?: Benjamin Silliman

441px-Benjamin_Silliman.jpg (33791 bytes)      Benjamin Silliman was more than just a mineralogist whose career merited his name being lent to a new mineral.  He was accomplished in science, the law, and education, all of which drove his ambitions.   As one of the first American professors of science at Yale, he began lecturing on chemistry in 1804, when Thomas Jefferson, our third President, resided in the new White House in Washington, D. C.  There were 15 stars on the flag and States of the Union.  And, the ink on the 1803 Louisiana Purchase document was still fresh.  Our early days of a national science were ready for many firsts.

(Left): Portrait of Benjamin Silliman, Sr. courtesy of

     His many amazing “firsts” propagated his fame even more so.  He was first to fractionate
petroleum distillates, first to publish a scientific account on American meteorites, and first to
discover new minerals and compositions.  We still marvel at most of these phenomena today.

     Professor Silliman founded the American Journal of Science (1818), and was named by the
U. S. Congress to membership in the National Academy of Sciences as a Charter Member in
1863.  (Source: "Founding of the National Academy of Sciences", NAS website)

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Silliman statue at Yale
courtesy of
15-star U. S. Flag at the time of Silliman's first lecture
Courtesy of Mark Sensen, FOTW

     In a time when both America and the science of Geology were new, a bright star of invention
shone ever so brightly.  He brought with him from his studies at the University of Edinburgh the
bases of European geology, as set forth by Hutton, Lyell, and others.

     He was born three years after George Washington and the Continental Congress signed the
Declaration of Independence, and was an elderly supporter of Abraham Lincoln.  He studied
after the fathers of modern geology, James Hutton and Charles Lyell, and was contemporary
(and father-in-law) to James Dwight Dana.  In his 85 years, this noted professor laid a solid
foundation for American education in Geology, and our national advancement in science.   This
author would call him the ‘Father of American Geology Education’.

danas.jpg (46286 bytes)      Dana, who built somewhat upon the work of his father-in-law, Benjamin Silliman, has his revised and updated work still used today in 21st century college classrooms.  I still refer to my 19th edition from 1977, which was assigned by my Geology Professor, Dr. Peter Leavens (like Professor Silliman, a Yale Graduate) at the University of Delaware.


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Here are two modern editions of Dana's Manual of Mineralogy.  One book includes a CD-ROM!  (
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     His legacy lives on in the expansive scientific achievement of his son Benjamin, Jr. and son-in-law J. D. Dana, also in his extensive collection of North American rocks, minerals, and fossils in Yale’s Peabody Museum, founded just two years after his death.  There were only 35 stars on our flag when our illustrious professor finished his earthly work. (Source: "Benjamin Silliman" Peabody Museum of Natural History, Yale University)

     Professor Emeritus Silliman’s name is remembered in honor at Yale’s Silliman College, Mt. Silliman and Silliman Lake in Sequoia National Park, by the State of Delaware, and as our favored mineral this month: Sillimanite.  His mineral name was grandfathered in 1959 by the International Mineralogical Association (IMA).

(Source: "Sillimanite" at

(Top): 35-star U. S. Flag, 1863-65,  Rick Wyatt, FOTW

(Bottom): 50-star U. S. Flag,
1959-, Joe McMillan, FOTW

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     Professor Emeritus Silliman’s name is remembered in honor at Yale’s Silliman College,
Mt. Silliman and Silliman Lake in Sequoia National Park, and our favored mineral this month:
Sillimanite.  His mineral name was grandfathered in 1959 by the International Mineralogical
Association (IMA).

(Source: "Benjamin Silliman" at
(Source: "Benjamin Silliman" and "The Collections: Mineralogy" Peabody Museum of Natural
                   History, Yale University)

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     Though Sequoia National Park is our nation’s second oldest park, it touts residence of four of the five world’s largest sequoias.  From scenic meadows one can view high ridges, resolving one’s view up to Mount Silliman (11,000’+).  In fact, the first national park (Yellowstone) wasn’t named until 1872, seven years after his death, so; he preceded a landmark American first.

(Source: "Sequoia & Kings Canyon National Park: What's in Each Area, Fall", U. S. National Park Service website)

View of Mt. Silliman
Courtesy of Mike Ostby ©2003


On a mission

     In 1805, at age 26, Chemistry Professor Benjamin Silliman was dispatched by Yale
University on a mission to the University of Edinburgh.  His objectives were twofold: one, to
procure laboratory equipment, and two, to experience the large-scale lecture methods first
developed by Professor Thomas Charles Hope.  Silliman succeeded on both counts with the
personal direction of Hope.  In order, Hope’s firsts in lecture methods translated into American
firsts at Yale.  (Source: "Thomas Charles Hope, MD, FRSE, FRS (1766-1844)" by W. P. Doyle,
University of Edinburgh website)

     Therefore, it is appropriate that “The First State, Delaware” adopted his namesake as it’s
official state mineral.  Delaware was the first state to ratify the Constitution.

     During Silliman’s time, a new nation needed an identity.  With new traditions still young,
Americans seemed to want a deeper anchor in the New World.  Geology gave it one.

     With scientific discoveries, such as those recounted from the 1804-06 Lewis and Clark
expedition, ancient histories could be devised, thus allowing old ground to act as the anchorage
into which a common identity could be instantly fastened.

     Writers, such as Fisher, Novak, and Bedell all agree that geology grounded the American
psyche, both literally and figuratively.

     To that which lied beneath the soil and majestically above, both science and art co-developed,
in overlapping, fashion a means to communicate grandly scaled panoramas in printed and painted
media to fuel the emerging American imagination.  These mega-views inspired the public’s evolving
awareness as could culminate in an image of a cultural grandeur over that of an older and esteemed
European worldview.  One could read a plethora of fresh books and news accounts on the subject,
as well as view large mural canvases by famous landscape artists of the times.

     Silliman’s publications were as enthusiastically received as those of Dana, Hitchcock, and
Lyell, as well as other pre-eminent scientists of his day.  In fact, those of his and his family
members have been revised, and are still referred to today in college classrooms.

(Source: "The Geological Revolution in American Time", Thomas M. Allen, University of Richmond)


Chemistry & Science

     Under our topsoil, and eroded on creek banks, the average observer can spot surface
Sillimanite.  Almost as common as quartz in one New Castle County, Delaware location,
this Nesosilicate offers us a glimpse into our local earth's past and present.

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Prismatic brown crystals of Sillimanite Sillimanite closeup showing striations parallel to the "C" axis
Photos by and courtesy of Stan Celestian, Glendale Community College (now ASU West)


Delaware Sillimanite Geology  

     Formed in the Delaware Piedmont, Sillimanite is a common occurrence in north-western
New Castle County.  Found in some streams, rounded stones exhibit a brown weathered rind,
like that of a potato skin.

     One could find large boulders just feet from our clubhouse at Greenbank Mill, Price’s Corner.

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     According to Delaware Geological Survey Open File Report #38 from June 1995, a core sample (#24895) from 45’ down on Centerville Road, near Prices’ Corner, contains “Bundles of sillimanite needles, blades, and large prisms clustered with biotite”.  (The Wilmington Complex contains other minerals, some of which may become future MOTM this year.)

(Source: "DGS Field Report No. 38, 1995", DGS website)

Delaware Geological Survey Building
Photo by Ken Casey

     Per geologists of the DGS and USGS, the Wilmington Complex is extremely ancient.  In an
article called "America's Volcanic Past: Delaware", rocks of the Delaware Piedmont age from
about one-half billion to 1.2 billion years old.  Sillimanite is included in the underlying strata as
component of the Delaware Gneiss.

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Creekbed area strewn with Sillimanite at BSP Closeup of Sillimanite boulder at BSP
Photos by Ken Casey

     Delaware Sillimanite was formed in a high-temperature metamorphic environment above
550 degrees Celsius over a prolonged time, thus producing coarse grains.
(Source: "Delaware State Mineral, Sillimanite", William S. Schenck, DGS website)

     Brandywine Springs sillimanite is 30% fibrolite by volume.  It is an index mineral in this
type of metamorphism.

(Source: "TEM Investigation of Lewiston, Idaho, fibrolite: Microstructure and grain boundary
, Lee, Banfield, Kerrick, American Mineralogist website)

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BSP Sample #1: Fibrolitic Sillimanite BSP Sample #2: Fibrolitic Sillimanite
Samples courtesy of Fran Poniecki, Photos by Ken Casey

     When broken, many specimens reveal parallel fiberlike crystals, resembling a wood-grain
texture.  Pertinent colors include white, gray, tan, and green.

     On a microscopic level, the crystallography of Sillimanite as Al2SiO5 can be viewed as
both thin-section under the microscope, and as theoretical drawings of their dynamic structure. 

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Sillimanite in thin-section from Moose River, NY
Photomicrograph courtesy of Dr. Robert S. Darling
Two-layer atomic bonding model of Sillimanite
Graphic courtesy of Professor Stephen Dutch

     Sillimanite consists of chains of aluminum octahedra linked by alternating aluminum and
silica tetrahedra.  Goodness knows, before 21st century computer modelings, some of us
20th century geology students struggled with self-made cardboard and glue cut-outs of these
polyhedra--assembling them to visualize Sillimanite's structure.  My first attempt was around
1977, when coincidentally, the DMS was rallying to propose the naming of an official state

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     Nature favors the formation of Sillimanite over Kyanite or Andalusite, when temperatures and pressures together are the highest.

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Kyanite-Andalusite-Sillimanite Phase Diagram
Graphic by Ken Casey
Eroded stream boulders of Sillimanite at BSP
Photo by Ken Casey


Brandywine Springs Park

BSP_postcard.jpg (34519 bytes)      Home to George Washington’s Council Oak and the historical site of Camp DuPont and Brandywine Springs Park, this location also contains stream deposits of Sillimanite. Though not for the taking, one can observe the geology that hosts these wonderful boulders!

(Left): Postcard of Brandywine Springs Park circa 1910

     This author was fortunate to receive a guided tour of the park by Lonzi, one of the
Friends of Brandywine Springs Park.  He showed me the spring head (which still flows today),
the ancient council oak tree (still on site), and the remnant foundations of the late 19th and
early 20th century amusement park (1886-1924).  Along with the monthly archaeological
activities that are conducted on site, Lonzi pointed out to me the challenges of preventing
erosion, the maintenance of hiking trails, and importance of safety concerns, such as not
climbing down the stream banks.

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Brandywine Springs Park Sign State of Delaware Historical Marker N.C. 79:
"Camp DuPont"
Photos by Ken Casey

     Interestingly enough, Rear Admiral Samuel F. DuPont's encampment at Brandywine Springs,
from May 1861 to October 1862, coincided almost with the time that Professor Silliman supported
President Lincoln.  This park contains a large deposit of Sillimanite!

     Lonzi, as a park steward, was also familiar with some of the geology of his park.  He showed
me examples of garnets, quartzes, and best of all, Sillimanite.  In the interest of this article, he
showed me the stream deposits of Sillimanite.  He would like me to let you know that collecting
is prohibited here, so please enjoy the park space responsibly.

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Bridge over creek at BSP Creek boulders, some are Sillimanite
Photos by Ken Casey

     I have been visiting this park for about 30 years now.  Sadly, it is no longer an amusement
park.  It has remained naturally unchanged, yet with improved facilities, and does meet the fun
and recreation needs of its many visitors.  I like how it is setup as a clean, excellently maintained
family park.  Please do visit, if you are in the New Castle County, the first county of the First State.

Attractions adjacent to Brandywine Springs Park:

Historic Greenbank Mill (our clubhouse)
The Wilmington & Western Railroad


Two Museums of Note

     One can find Sillimanite prominently displayed in the collections of two important museums of
note: The Yale Peabody Museum and The Irénée DuPont Mineral Museum at the University of

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Sillimanite Boulder at the U. of D.
Photo courtesy of William S. Schenck, DGS
Museum plaque DMS fieldtrip/meeting at the Irénée DuPont Mineral Museum in 2006 (Ken Casey)

     Yale's collection contains 46 specimens from the east coast of the U. S. and from around the
globe.  A comprehensive list is below:

Sillimanite can be found in Connecticut, near Yale at Guilford, New Haven County.  Also, at:

Norwich, New London County, Connecticut
Willimantic, Windham County, Connecticut
North Woodstock, Windham County, Connecticut
Winsted, Litchfield County, Connecticut
Haddam, Middlesex County, Connecticut
Westbrook, Middlesex County, Connecticut
Orange, Franklin County, Massachusetts
Winchester, Middlesex County, Massachusetts
Wales, Hampden County, Massachusetts
Monroe, Orange County, New York
Media, Delaware County, Pennsylvania
Seneca, Oconee County, South Carolina
Burke County, North Carolina
Brandywine Springs, New Castle County, Delaware
Norway, France, Madagascar, Kenya, Germany, Czechoslovakia, and Australia
(Source: Yale Peabody Museum Catalog Service, Query: Sillimanite)



     The major uses for Sillimanite are industrial and aesthetical.  By using our car, visiting a
park,or by wearing jewelry, we can appreciate this aluminum silicate wonder. 

     Engine spark plug manufacture requires the addition of an insulating compound.  Pulverized
Sillimanite is employed here.

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(Top): Sillimanite boulder at BSP (Ken Casey)
(Bottom): Newly manufactured spark plug (
Champion Spark Plug Novelty item
(Photo by Terry Schwartz)

     As mentioned above, Sillimanite boulders can act as a natural backdrop to outdoor recreation. 
Perhaps one of you might paint a picture or photograph the park for either geologic study, or for
art's sake.  Happy trails!

     The second aesthetic use of Sillimanite is as a gemstone--there's more below.



6yy-catseye-sillimanite-newstar45.jpg (79606 bytes)       "Sillimanite is suitable for lapidary work and under the name Fibrolite..."

(Source: "Delaware Geology: State Mineral: Sillimanite", State of Delaware website)

     It is the fibrolite components that contributes towards the optical dispersion noted in cabochon gems cat’s-eye effect.  The coarser grained material is more suitable to faceted gemstones.

Cat's-eye Sillimanite Cabochons
Photo courtesy of Roger Weller, Cochise College

     On the jewelry-side, faceted fine gemstone material can yield some of the most beautiful
brown-green-yellow color combinations.  I understood this use better by enjoying a DMS
club meeting program on faceting, presented by our own Tom Pankratz.  One of Tom's hobbyist
goals is to facet at least one of every major gemstone known to man.

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Tray of Faceted Gem Sillimanite
Close-up of Faceted Sillimanite Gem!
Photos courtesy of Roger Weller, Cochise College ©2004

     Another DMS member, Bob Todd, is an avid collector of the Al2SiO5 trimorph.  Bob specializes
in Sillimanite-Kyanite-Andalusite.  He purchases gem rough and cut stones.  I suggested to Bob
at one meeting, that he might consider faceting a very large gem, and call it "The Star of Delaware". 
Who knows what we might see from Bob and Tom in the future.

30th Anniversary as Delaware's Official State Mineral  

     The 1970s heralded celebration of our nation's bicenntennial.  In 1977, our club, the
Delaware Mineralogical Society, proposed an official State recognition of a mineral: our
own Sillimanite.

     Before 1830, during Silliman’s tenure at Yale, Delaware geologists found remarkably pure
boulders of Sillimanite in the alluvium of Brandywine Springs. 

(Source: "Delaware Geology: State Mineral: Sillimanite", State of Delaware website)

     Dana listed this locale in his 6th edition “Dana’s System of Mineralogy (1892).  These two
events, along with a late 20th-century appreciation of this mineral, culminated in the official
celebration, which we observe today.

(Source: "Delaware State Mineral: Sillimanite", William S. Schenck, DGS website)

     Delaware sillimanite’s gem properties kept it in the public eye for decades, as hobbyists
lapped and faceted it into chatoyant cabochons and sparkling gems.  This attention, along with
its impressive history, prompted our club to propose to the Delaware General Assembly in 1977,
that it be named as the official state mineral.  The Act was passed and written into law as our
Delaware Code Title 29 § 310.  We celebrate this anniversary on March 24th.  So, mark your
calendar, and wherever you are, please toast with us.  Cheers!


State Government

General Provisions


§ 310. State mineral.

The official state mineral is sillimanite.
(61 Del. Laws, c. 21, § 1.)


(Source: "Title 29 § 310", State of Delaware Official website)



Irénée DuPont Mineral Museum, University of Delaware
Peabody Museum of Natural History, Yale University
Friends of Brandywine Springs Park, New Castle County, Delaware
State of Delaware Division of Historical & Cultural Affairs
Historical Society of Delaware
DelDOT Projects: Archaeological Exploration and Historical Preservation in Delaware
Delaware Greenways
International Mineralogical Association


Members' Gallery

Here is where DMS Members can add their nice Sillimanite photos to share with us.

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IMGP0192.JPG (246121 bytes) (Top, Left): DMS Junior Chairperson, Fran Poniecki, shows some Girl Scouts samples of Sillimanite at our October 2006 Educational Event at Greenbank Mill

(Top, Right): Labeled Sillimanite from the Red Clay Creek area

(Bottom, Left): Tray of Sillimanite sample which Fran shared with the Scouts

(Photos by Ken Casey)


Until Next Time

     We hope you have enjoyed our quaint visit to Sillimanite.  Please join
us next month, for another article, and we shall journey together!
     Until then, stay safe, and happy collecting. hardhat2a.gif (5709 bytes)



Article Contributors

Fran Poniecki, Junior Chairperson, The Delaware Mineralogical Society


Photo & Graphics Credits

I would like to gratefully acknowledge the generous contributions of our fellow Sillimanite
enthusiasts, collectors, authors, curators, professionals, and club members who made this
work possible.  Thanks.

World Flag Database
Flags of the World, Mark Sensen, Rick Wyatt, Joe McMillan, Mike Ostby
Stan Celestian, Instructor, Glendale Community College (now ASU West)
Dr. Robert S. Darling, Professor of Geology, SUNY Cortland Geology Department
Professor Stephen Dutch, University of Wisconsin, Green Bay
William S. Schenck, Delaware Geological Survey
Roger Weller, Cochise College, Magnus Manske
Terry Schwartz

©2007  All contributions to this article are covered under the copyright protection of this article
and by separate and several copyright protection(s), and are to be used for the sole purposes of
enjoying this scholarly article.  They are used gratefully with express written permission of the
authors, save for generally-accepted scholarly quotes, short in nature, deemed legal to reference
with the appropriate citation and credit.  Reproduction of this article must be obtained by express
written permission of the author, Kenneth B. Casey, for his contributions, authoring, photos, and
graphics.  Use of all other credited materials requires permission of each contributor separately
Links and general contact information are included in the credits above, and throughout this article.
The advice offered herein are only suggestions; it is the reader's charge to use the information
contained herein responsibly.  DMS is not responsible for misuse or accidents caused from this
article. All opinions, theories, proofs, and views expressed within this article, and in others on this
website, do not necessarily reflect the views of the Delaware Mineralogical Society. 

Suggested Reading:

Life of Benjamin Silliman, M D., LL., D., Late Professor of Chemistry, Mineralogy, and Geology in Yale College. Chiefly from His Manuscript Reminiscences, Diaries, and Correspondence.: Vol. 2, Michigan Historical Reprint Series


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   About the Author:  Ken is current webmaster of the Delaware Mineralogical Society.  He has a diploma in Jewelry Repair, Fabrication & Stonesetting from the Bowman Technical School, Lancaster, PA, and worked as jeweler.  He has also studied geology at the University of Delaware.  And, he is currently a member of the Delaware Mineralogical Society and the Franklin-Ogdensburg Mineralogical Society.  E-mail:

Invitation to Members


Want to see your name in print?  Want to co-author, contribute, or author a whole Mineral of the Month article?  Well, this the forum for you!

And Members, if you have pictures, or a story you would like to share, please feel free to offer.  We'd like to post them for our mutual enjoyment.   Of course, you get full photo and author credit, and a chance to reach other collectors, hobbyists, and scientists.  We only ask that you check your facts, give credit where it is due, keep it wholesome for our Junior Members watching, and keep on topic regarding rockhounding.

You don't even have to be experienced in making a webpage.  We can work together to publish your story.  A handwritten short story with a Polaroid will do.  If you do fancier, a text document with a digital photo will suit, as well.   Sharing is the groundwork from which we can get your story out there.

Our club's webpages can reach any person surfing the net in the world, and even on the International Space Station, if they have a mind to view our website!

We are hoping for a possible tie-in to other informative programs upon which our fellow members might want to collaborate.  Contact any officer or board member with your suggestions.

Our next MOTM will be a surprise. For the future, we are waiting for your suggestions.  What minerals do you want to know more about?

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Most of the Mineral of the Month selections have come from club fieldtrips and March Show Themes, and from inspiring world locales. thus far.  If you have a suggestion for a future Mineral of the Month, please e-mail me at:, or tell me at our next meeting.



Past Minerals of the Month
Note: These articles are as originally published, and will be updated.
December 2006 Mineral of the Month: Hedenbergite by Karissa Hendershot
November 2006 Mineral of the Month: Brandywine Blue Gneiss
October 2006 Mineral of the Month: Spessartite by Karissa Hendershot
September 2006 Mineral of the Month: Native Silver
August 2006 Mineral of the Month: Kryptonite
July 2006 Mineral of the Month: Azurite
June 2006 Mineral of the Month: Pyromorphite
May 2006 Mineral of the Month: Tsavorite by Karissa Hendershot
April 2006 Mineral of the Month: Variscite
March 2006 Mineral of the Month: Petrified Wood, Part II
February 2006 Mineral of the Month: Petrified Wood, Part I
January 2006 Mineral of the Month: Strontianite by Karissa Hendershot
December Mineral of the Month: Clinozoisite
November Mineral of the Month: Bismuth
October Mineral of the Month: Wulfenite by Karissa Hendershot
September Mineral of the Month: Turquoise
August Mineral of the Month: Peridot
July Mineral of the Month: Ruby
June Mineral of the Month: Antarctic Fluorite
May Mineral of the Month: Dolomite, Part 2
April Mineral of the Month: Dolomite, Part 1
March Mineral of the Month: Calcite
February Mineral of the Month: Agate
January Mineral of the Month: Fluorite
December Mineral of the Month: Pyrite
November Mineral of the Month: Stilbite  
October Mineral of the Month: Celestite   

This page last updated:  February 26, 2017 07:36:00 PM