Compiled by R. V. Dietrich, Professor Emeritus
Illustrations by Jeffrey Scovil, Dick Dietrich and
Comments and suggestions are welcome
instilled in me an insatiable curiosity about rocks.
The "compiled by" preceding my name on the "title page" seems appropriate because much of the included information is available in previously published material.
To all those whose information is incorporated in this presentation I acknowledge my debt of gratitude. Two groups in particular warrant special mention: the chroniclers who have helped preserve legends, anecdotes etc. about gemrocks; and those who, during the last several years, have recorded so many observations about gem materials in the GemNews and Gem Trade Lab Notes columns published in "Gems & Gemology." Although several of the legends I mention are recorded by Kunz (1913) and Merrill (1922), many are tidbits I have read or heard over the last approximately 70 years (where or from whom I do not recall), and I have not searched for possible sources for those tidbits in order to cite published sources (if, indeed, such exist) in this document. So far as the "Gems & Gemology" reports are concerned, several served to introduce me to gemrocks I might otherwise have overlooked.
Frederick H. Pough, Frances S. Dietrich, Dona Dirlam (Director of Liddicoat Geological Library, Geological Institute of America) and James E. Shigley (Director of Research, G.I.A.) critiqued the complete original manuscript, and Kurt R. Dietrich (Professor, Ripon College), Richard S. Dietrich (Director of the Lay Institute, Columbia Seminary), Daniel E. Kile (U.S. Geological Survey), and George W. Robinson (Curator, A.E. Seaman Mineral Museum, Michigan Technological University) critiqued parts of the manuscript; each of these people made suggestions that led to improvements. While I was compiling the information: David Ginsburg (Reference Librarian, Central Michigan University) formulated and carried out searches of different data bases; Margaret Dodd, of the Document Access Department of the CMU library sought out sources and obtained publications, several rather obscure, through interlibrary loans; and Sue James of the Ellis Memorial Library, Port Aransas, Texas, obtained five articles through interlibrary loan services for me during our annual January-April sojourns in that area. The “others” noted on the “title page” as providing some of the included illustrations -- sent me between eighty and ninety photographs or slides or gave me permission to download illustrations from their web sites; each of these people and/or organizations is given credit in the appropriate captions. Jeffrey S. McDowell (Help Desk Manager, Information Technology, CMU) helped reformat a few of the llustrations downloaded from the internet. Emmett Mason (Professor Emeritus of CMU) graciously created the webpages and while doing so provided several suggestions that improved not only the appearance but also the content of this document.
Undoubtedly some rocks used as
gemrocks are not included in this document and noteworthy information
about those that are included is less than comprehensive. I shall
readers' directing my attention to such omissions for possible
in my planned, continual updating of this document. Updates and modifications made after
September 10, 2009 are preceded by two asterisks, one on either side of
a plus sign (*+*). These
are NOT included on the PDF copy in the CMU CONDOR Archive.
Although some gemrocks are rare, others are garden-variety stones that occur here and there the world over. All that is required by anyone who wants to fashion nearly any rock into a gemstone or attractive ornament is imagination and technical know-how. Indeed, during prehistoric times, beads, amulets and talismans were fashioned from an extremely diverse array of rocks (as well as minerals) and, today, the number and variety of rocks being used as gemrocks is ever increasing.
When selecting rocks for possible fashioning into gemstones or ornamental or functional items, most lapidaries, carvers et al. usually consider a rock's eye appeal first and then its workability and probable durability. In addition, some of them also look into the rock's history (etc.) to see if it has any associated lore or history that might make anything fashioned from it gain some additional appeal. They realize that these factors, except for workability, are important to anyone who markets and purchases the items they fashion. These aspects are covered directly or indirectly for most of the materials treated as the main entries in this document.
GEMROCKS versus GEM MINERALS
It seems only prudent to outline the mineralogic-petrographic basis whereby gem minerals and gemrocks were distinguished so far as deciding what materials should be covered in this document: The following definitions and analogy describe and clarify the basis used.
A MINERAL may be defined as a natural substance, generally inorganic, with a characteristic internal arrangement of atoms and a chemical composition and physical properties that are either fixed or vary within a definite range.
A ROCK may be defined as a natural solid composed of mineral grains, glass or a combination of mineral grains and glass. (And, following widespread tradition – but not accepted by all petrologists – coal and other naturally consolidated accumulations of plant and animal matter are also considered to be rocks.)
Relationships between rocks and minerals have frequently been clarified by reciting analogies: For rocks made up wholly or largely of minerals, I especially like the rock is to mineral as forest is to tree analogy. To examine it briefly: Some forests contain several species of trees just as some rocks contain several different minerals; other forests are made up almost wholly of trees of a single species just as other rocks are composed largely, if not wholly, of grains of only one mineral. In addition, just as the trees of a forest may be of different shapes and sizes, mineral grains of a rock may have diverse shapes and sizes. In any case, even though some forests and rocks are similar to other forests and rocks, and consequently may be classified together, strictly speaking each forest and each rock is unique.
Even with an understanding of the chief distinctions between rocks and minerals, some people who have only examined gemrocks macroscopically may find it difficult to to accept some of materials included in this document as rocks rather than minerals. Those most likely to be questioned include agate, chalcedony, the jades and a few or several others, depending upon the person's mind-set. The mental hurdle they most likely will have depends on the fact that these gemrocks are aggregates made up wholly or largely of extremely fine-grained microscopic particles of a single mineral, and consequently many of these rocks appear to be homogeneous -- that is to say, especially as relatively small pieces these rocks resemble parts of crystals or large mineral grains.
Fortunately, most of those who face this dilemma are willing, if not eager, to look at these rocks through high-powered microscopes or at photomicrographs or electron micrographs of them (See Figures I.A and B). And, once they have done so, it is quite apparent that each of these these materials comprises aggregates of innumerable mineral "micrograins," albeit wholly or largely of only one mineral. Thereafter, most of them are quite willing to accept "on faith" the fact that other similarly homogeneous materials described herein as gemrocks have also been found to consist of innumerable fine "micrograins," and thus also are rocks rather than minerals per se. To state it another way, they realize that these rocks are merely extremely fine-grained monomineral rocks akin to coarser grained monomineral, polycrystalline rocks such as anorthosites and larvikites, and many limestones, sandstones, marbles and quartzites -- i.e., the major constituents of these rocks occur as macrocrystals as well as the components of these rocks. For those that do not accept this concept, it seems that at least some of them think this viewpoint opens a "Pandora's box." My hope is that it will sharpen the thinking of those who deal with these materials.
Figure 1 A. Chalcedony. Photomicrograph (cross-polarized light; field of view heigth - ca. 4.8 mm across) of banded chalcedony, from south-central Saguache County, Colorado. As can be seen, this chalcedony consists of innumerable microscopic grains, virtually all of which are quartz. As noted in the discussion, this makeup, which is typical, is the basis of calling chalcedony a rock -- i.e., it is a microgranular, monomineralic rock. (© photo by Daniel E. Kile)
Fig. 1 B. Nephrite jade.
Photomicrograph (cross-polarized light; field of view heigth -
ca. 4.8 mm across) of nephrite jade from British Columbia,
Canada. This nephrite can be seen to consist of innumerable
microscopic grains, virtually all nephrite, which is a variety of the
tremolite-actinolite series of the amphibole group of minerals -- i.e., it (like all typical nephrite
jade) is a microgranular, monomineralic rock. (©
photo by Daniel E. Kile)
INFORMATION GIVEN FOR EACH GEMROCK
The chief gemrocks are treated in alphabetical order. Information, if known, is given for each gemrock under the following subheadings:
NAME OF MATERIAL --. Fr - French, Ger - German, Nor- Norwegian (which correlates well with other Scandinavian languages), and Rus - Russian (cyrllic letter version) common names are included in parentheses below the entry title name. For completeness, the equivalent names are given for materials whose names are virtually the same as in English. For some of the materials whose French, German, Norwegian or Russian names were not discovered during my search I suggest translations -- e.g., sang-de-poulet pierre as the French for chicken-blood stone; anyone who knows the correct terms for these listings may find my suggested translations amusing so I request that (s)he Please send me the correct terms for substitution!! No foreign language equivalents are given for goodletite, leopardite, leopard rock, and unakite because it seems unlikely they have been recorded in the foreign literature. If, however, anyone knows equivalents for any of these terms, I would be pleased to have them sent to me for inclusion in updated copy.
On the following line, the name of the entry material is repeated, and where appropriate, alternative spellings, chemical formulae for gemrocks comprising masses made up largely or wholly of a single mineral, and cross-references to closely related gemrock entries are noted.
DESCRIPTION -- Properties given are for the variety or varieties used as gemrocks -- i.e., the listed properties should not be considered necessarily to apply to all varieties of the named mineral or rock. Most of the listed properities can be seen by macroscopic examination and/or determined without using sophisticated equipment.
So far as the properties given, three aspects are particularly noteworthy:
1. Hardness, is given according to Mohs hardness scale (see the Glossary, Appendix B). For gemrocks whose mineral grains are too small to check individually, hardness should be checked by using a procedure opposite from that usually employed -- i.e., the gemrock should be used as the "scratcher" rather than the material being scratched. This approach is, for all practical purposes, required because many rocks made up of innumerable grains, however small, tend to become disaggregated and thus appear to have been scratched when one tries to scratch them even with, for example, hardness pencils of lesser hardness. On the other hand, if the material being to be checked is used as the "scratcher," in most cases it will scratch anything of inferior hardness, thus establishing its true effective hardness -- i.e., the hardness of its hardest constituent. IN ADDITION, Hardness should NOT be checked on polished surfaces because it might be destructive.
2. Specific Gravity, referred to as density in some publications, may be determined by weighing a material in air, weighing it again while it is submerged in water, and using the measured values to make simple calculations -- see the Specific gravity entry in the Glossary (Appendix B). Alternatively one can use the heavy liquid method also described in the Glossary. In any case, it should be kept in mind that specific gravity values one may determine for aggregates of single minerals are commonly lower than those given in books (and herein!) for these gemrocks; those values have been established by measuring single crystals and/or by calculations based on the compositions and structures of the given mineral. Whatever, Specific Gravity determinations are nondestructive for all except easily soluble materials of which there are very few among widely used gemrocks.
3. Double-checking: Anyone unsure of her/his identification of an "unknown gemrock," should have an expert, who has the necessary knowledge and equipment, identify the material.
OTHER NAMES – Examples of designations, such as varietal names, names based on localities, and trade names, are given under this subheading. – I say "examples" because it seems likely additional names would have been found had I searched through, for example, labels in museum and private collections and in "rock shops." -- Along this line, attention is directed to Mitchell's Dictionary of Rocks (1985) and Fourestier's Glossary of Mineral Synonyms (1999). -- In any case, with few exceptions, only names applied to material recorded or otherwise known by me to have been used to fashion gemstones and ornamental and curio stones are included. Some of the marketplace names, in particular, warrant inclusion of the following statement by Merrill (1922, p.179):
"Many of the names have been coined by the dealers for particular minerals [and gemrocks] for the evident purpose of increasing their sales. Many people who buy cheap gem stones under fanciful names probably would not buy the stones if they were offered under their true mineralogical [or petrographic] names."
USES -- With a few exceptions only examples of common uses as a gemrock are given -- i.e., most of the lists are not comprehensive because I am confident that readers can think of additional things that probably have been -- or at least could be -- fashioned from the given gemrock.
OCCURRENCE(S) -- One or more geological environment where the described gemrock material has been found to occur is noted. This information is provided with the hope that it may help anyone trying to find the rock in the field. It is a well known fact that many rock-to-geological environment associations occur at many worldwide locations, and it also is known that not all such locations have yet been discovered and/or recorded.
NOTEWORTHY LOCALITIES -- Only one or a few so-to-speak "famous" or well-known localities are listed. As a result of this delimitation, information given under this subheading will undoubtedly be less than what many readers desire. -- By way of explanation, my attempts to decide what localities should and should not be included were extremely frustrating. I decided on the approach used only after consulting a number of professional colleagues. For several of the gemrocks, however, additional localities are noted elsewhere in the entries --e.g., after listings under OTHER NAMES subheading. In addition, anyone interested in this kind of information should check the REFERENCE(s) cited and articles cited in the bibliographies given in those references. Also, one should check the literature for guidebooks and regional geological reports. Finally, John Sinkankas' three-volume "Gemstones of North America" and P.G. Perazzo's "Stone quarries and beyond" web site (see Appendix C for complete references) list localities for several of the gemrocks described in this document.
REMARKS -- The first remarks relate to the origin of the name of the gemrock and/or one or more of its varietal names or major mineral component(s) IF such seem especially noteworthy. Information about Treatments -- such as coating, dyeing, and impregnation -- used to change or enhance the appearance of the material are given for all gemrocks for which such information is known. Much of the rest of the remarks, if any, comprise miscellany such as one or more of my favorite bits of associated history, lore and/or anecdotes about the gemrock. For some entries, information is given about the gemrock's having been interpreted as occupying one or another position in Aaron's breastplate, as constituting one of the walls of the Heavenly City as described in the book of Revelation in the Bible, or being the official rock, mineral, gem (etc.) of some state or other civil entity. These "tidbits" are given as examples of the kinds of things one might wish to know if (s)he were to use something made from the given gemrock as a conversation piece.
Most of the legends (etc.) recited are only examples of innumerable ones recorded in the literature. (Along this line, several attributes correlating gemrocks with human physical and mental conditions, activities, and aspirations appear in the literature of metaphysics and can be seen on several web sites. A large percentage of these "relationships" appear to be based on numerology or to have been concocted relatively recently as marketing devices; with a few exceptions, these "late-comers" are not recorded on this web site.) For the record, my feelings about these attributes agree, for the most part, with those of Dr. Kunz (1913), who wrote the following:
"The writer has always sought to investigate anything strange and apparently unaccountable which has been brought to his notice, but he can truly say he has never found the slightest evidence of anything transcending the acknowledged laws of nature. Still, when we consider the marvelous secrets that have been revealed to us by science and the yet more wonderful things that will be revealed to us in the future, we are tempted to think that there may be something in the old beliefs, some residuum of fact, susceptible indeed of explanation, but very different from what a crass sceptism [sic] supposes it to be. ...
"Auto-suggestion may ...afford an explanation of much that is mysterious in the effects attributed to precious stones, for if the wearer be firmly convinced that the gem he is wearing produces certain results, this conviction will impress itself upon his thought and hence upon his very organism. He will really experience the influence, and the effects will manifest themselves just as powerfully as though they were caused by vibrations or emanations from the material body of the stone."
SIMULANTS -- Both natural and manufactured materials are used as simulants; names of the manufactured materials are preceded with three asterisks (***). Whereas names on labels and catalog listings of some items clearly indicate their gemrock constituents to be simulants -- e.g., the gemrock names are modified by adjectives such as faux ..., man-made ..., synthetic ... -- far too many marketplace designations are gimmicks. Materials indicated as faux (etc.) ... are not included unless their compositions are known and seem noteworthy. One or more property that may serve as an aid to distinguish each simulant from the true material is given -- enclosed in square brackets ( [ ] ) -- at the end of each of the simulants.
REFERENCE(S) -- Selected general references -- or the indication "No general reference" plus or minus one or more references that may be of particular interest -- are given to aid readers wishing to pursue additional information about the given gemrock. The complete reference for each is given in Appendix C.
Selecting the cited references was difficult for some of the gemrocks. Decisions as to what reference(s) should be included related to the following considerations: The probable availability of the publication in libraries and/or bookstores; the coverage including illustrations and number of cited references that might aid anyone wishing to persue additional information about the given material; the level of treatment thought most likely to be reader friendly for a majority of the anticipated audience of this document; and their up-to-dateness. Nonetheless, some of the listed references are not easy to find; some have few, if any, good illustrations and/or bibliographies; a few contain some highly technical data; and some are relatively old. This last aspect is especially noteworthy because some more recently published references could have been cited for some of the entries; my choices of older references reflects the fact that I am a firm believer that more recent does not necessarily mean better. One more thing: With a few exceptions, mineralogy and petrology "textbook/references" are not listed; it "goes without saying" that some of them are well worth consulting so far as getting additional information about some of the gemrocks and/or their constituents.
A. ADDITIONAL GEMROCKS is a tabulation of gemrocks, not included as main entries; a reference and remarks are included for each.
B. GLOSSARY of geological, mineralogical, petrological and technical terms with definitions that indicate how each is used in this document.
C. SELECTED REFERENCES, a few of which are
annotated, includes general references as well as the cited
ETYMOLOGY, general statement re the derivation of the term AND a
list of terms applied to amber in several languages
GEMROCKS, a brief outline of how fashioned rocks, some of
which are described as gemrocks on this website, may be used in
archaeological studies; this example is based on a study of
prehistoric Caribbean sites.
This index, which is rather comprehensive,
followed by links to Gemrocks Entries in which the terms appear. The
links do not, however, go to the indicated term; a search procedure,
such as Ctrl-F, may be used to find the term within the
Entry. Index Terms which are also Gemrocks Entries are
capitalized and in bold face type -- e.g., Agate.
Titles of named carvings and sculptures are
enclosed in quotation marks. Unnamed carvings and ortherwise fashioned
pieces, photographs of which are included in Entries, are indexed under
their particular genre -- e.g., Zimbabwe carving and
Victorian necklace -- and/or included under terms such as beads,
cabochon, egg, pendant, and sphere.
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Latest revision/update: 6 September 2013
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