BRECCIA


BRECCIA

A. Breccia cabochons (largest one, greater dimension - ca. 5 cm). Smithsonian Institution. (photo by L. Bolton)

B. Breccia bowl (greatest dimension - ca. 10 cm). Smithsonian Institution. (photo by B. Chromy)

DESCRIPTION: Breccia is the name widely applied to a diverse group of rocks that consist of angular, greater-than sand-size fragments surrounded by a finer grained matrix. The diversity depends upon the fact that breccias have several modes of origin, and consequently, both the fragments and the finer grained matrices may be of just about any composition.  Consequently, the properties of breccias differ markedly from rock to rock -- see REMARKS.

OTHER NAMES: A few breccias have extents that have led to their being named according to the scheme used for stratigraphic units (see Appendix B, Glossary). Two examples are the Mackinac Breccia, which is well exposed in St. Ignace and on nearby Mackinac Island, Michigan; and the Zopilote Breccia of the Permian basin of west Texas.   The following designations are, for the most part, marketplace names:

USES: Beads; cabochons for various pieces of jewelry, particularly relatively large ones such as brooches and belt buckles;  paperweights and  bookends;  diverse vessels (see Figure B);  carvings;  etc.

OCCURRENCES: Breccias occur in several diverse environments where angular rock fragments have been enclosed in a finer grained matrix and lithified -- see REMARKS.

NOTEWORTHY LOCALITIES: See those given under the OTHER NAMES subheading.

REMARKS:  The term breccia, according to OED, has its roots in the Old Teutonic brekan (to break) and comes from the Italian where it was "used in the name Breccia Marble before its separate use [as a rock name] in Geology."  The Italian apparently meant in a literal sense "gravel or rubbish of broken walls" (ibid.).    

Geologically, most breccias are either lithified rubble -- e.g.,lithified sediments or pyroclastic deposits that consist to a noteworthy extent of pebble- to boulder-sized angular fragments -- or were formed by crushing and grinding within fault zones. These breccias are generally referred to as sedimentary breccias and cataclastic (or fault) breccias, respectively.  However,  many breccias, including those used as gemrocks, have been formed as the result of other processes -- e.g., desiccation, which involves cracking as the result of dehydration or drying an subsequent filling of the cracks to give the overall mass a coherence.  Indeed, in his "Dictionary of Rocks" (1985),  my good friend Dick Mitchell (1929-1989), to whose memory I dedicated "Minerals of Virginia, 1990," has 62 entries (more than two pages!) of names that he found applied to breccias;  most relate to genesis, a few are only descriptive.

In any case, it seems very likely that breccias other than those listed under the OTHER NAMES subheading have been or in the future will be used at least locally as the rough for fashioning ornamental and curio stones.  And, whatever their origins or compositions, I suspect several will be given so-to-speak marketplace names rather than the names on Mittchell's list.  Also, I suspect that even though most breccias used as gemrocks will be chosen because they are considered attractive, some will be fashioned and marketed because of their potential appeal as conversation pieces -- e.g., impact breccias


SIMULANTS:  I know of nothing that has been produced and represented falsely as breccia or even substituted for breccia so far as its use in fashioned gemrock pieces.  However, considering how closely some terrazzos resemble breccias, I suspect someone will (perhaps already has) made terrazzo masses with dimensions such that they could be fashioned into items that resemble those fashioned from breccia....  It would seem that such masses of terrazzo could be carved or otherwise fashioned at least as easily as similarly appearing breccias.  Furthermore, it would seem that terrazzo could even be cast into desired forms. - [If the ingredients for the terrazzo were scrupulously chosen to avoid detection, distinguishing such pieces from those fashioned from natural breccias could be virtually impossible, at least by macroscopic means.].

REFERENCES:
No general reference.

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R.V. Dietrich 2015
Last update: 9 July 2005
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