( Fr- amphibole-riche roche; Ger- Amphibol-reich gestein; Nor- stein rik på amfibol;
Rus- [= amphibolite] )


A. Amphibole-rich rock cabochon (greater axis - 1.7 cm) from Converse County, Wyoming.  R.V. Dietrich collection. (© photo by Dick Dietrich)

B & C. "Dianite", also known as Siberian blue nephrite, cabochons (height of both - 2 cm) from Sakha (formerly Yakutia) in the Murrun Mountains of central Siberia, Russia. (© photo by courtesy of Webminerals s.a.s.,

DESCRIPTION: Amphibolite and amphibole gneiss are the most common amphibole-rich rocks.  Both are foliated metamorphic rocks and many are difficult to distinquish macroscopically one from the other.  By definition, both consist largely of an amphibole;  they differ in that   amphibolite has plagioclase feldspar as its second most abundant constituent whereas amphibole gneiss has quartz in that secondary role.
    Colors - medium green to nearly black, commonly also containing macroscopically visible grains that are off-white (quartz or plagioclase, typically andesine or high-calcium oligoclase) to nearly colorless (quartz);  see, however, the statements below about  the colors of some of the less common  amphibole-rich rocks that have found use as gemrocks.
H. (i.e., effective hardness) 6 - 7;  the hardnesses of their chief constituent minerals are actinolitic amphibole - 6 - 6½; plagioclase feldspar - 6 - 6½;   and quartz - 7)
    S.G. 2.5 - 2.75
    Light transmission - opaque.
    Breakage - irregular to conchoidal
    MiscellanyNoteworthy properties for amphibole-rich rocks that have found greater use as gemrocks than either the amphibolites or amphibole gneisses follow, in alphabetical order:

        "Blue equivalent of anthophyllite" rock is said to exhibit schiller and exhibit an appearance that is characterized as labradorite-like.  I have not seen this rock and know of no additional descriptive information about it that seems noteworthy here.

       "Ghost jade," lacks the fine-grained, felted structure required to be considered nephrite; consequently, it is not a true jade according to widely accepted nomenclature.   It is composed largely of actinolite and tremolite or nephrite (Johnson et al., 2000, p.68).  Colors - dark, medium, and light green plus white and off-white;  H. 6 - 6½;  S.G. ~ 3;  subtransparent to subtranslucent;  chatoyant, apparently a manifestation of the fact that a large percentage of at least the lighter colored amphibole is fibrous.

        Nephrite -- see NEPHRITE entry.

        Nuummite and a similar rock from Wyoming -- see Figure A -- are relatively coarse-grained nonfoliated amphibole-rich rocks.  Colors - brown (including golden brown), green and nearly black;  H. 5-6;  S.G. ~ 2.8;  individual grains, which are typically a few millimeters in greatest dimension, have dull to subvitreous lusters, are subtranslucent to opaque, and commonly exhibit golden brown, rose-red, silver-gray and/or greenish iridescence.

         "Siberian blue nephrite" is a massive blue rock, commonly gneissic "composed of a submicroscopic mixture of quartz, tremolite, and another (blue) amphibole, potassium magnesio-arfvedsonite" (Johnson et al., 2000,p.68) -- see Figures B & CAlthough the amphiboles constitute the major portion of this rock, their compositions are such that this rock should not be considered to be nephrite or jade.  Colors - "mottled saturated 'royal' or 'lapis' blue to a mottled desaturated grayish blue ... similar to 'denim' lapis" (op. cit.);   H. 6-7;  S.G. ~2.9;  subtranslucent.    

OTHER NAMES: Some amphibolites and amphibole gneisses constitute geological units that have been named according to the scheme used chiefly for sedimentary stratigraphic units (see Appendix B, Glossary) -- i.e., ... Amphibolite with the ellipsis replaced by a geographic name.  An example is the Ketchepedrakee Amphibolite of the Piedmont-Blue Ridge province of Alabama.

Special names, other than those already mentioned, that have been given to amphibole-rich rocks used as gemrocks follow:

USES:  Each of the listed amphibole-rich rocks has been fashioned into attractive cabochons and miscellaneous shaped pieces for jewelry.   Amphibolites and amphibole gneisses of some rock sequences are rather attractive and have been used, albeit rarely, as gemrocks -- e.g., in the past, for beads; more recently as paperweights and bookends.

OCCURRENCES: Most amphibolites and amphibole gneisses occur in terranes with metamorphosed sedimentary and/or volcanic rocks as the chief bedrock -- e.g., the Wyoming rock occurs in a weathered gneiss-schist unit, near its contact with a peridotite mass.

NOTEWORTHY LOCALITIES:  The "blue . . . anthophyllite" rock is from near Butte, Silverrod County, Montana.  "Ghost jade" comes from the vicinity of Yerington, western Nevada (Johnson et al., 2000, p.68).  Nuummite occurs in the Nuuk region (including offshore islands) of southwestern Greenland (Appel and Jensen, 1987) and in the Sahara Desert of central Mauritania (Renfro, 2011). The illustrated amphibole-rich rock occurs southwest of Douglas, Converse County, Wyoming (Dietrich et al., 1988).  "Siberian blue nephrite" is recovered "as a byproduct of nephrite jade mining in Sakha (formerly Yakutia) in the Murrun Mountains of central Siberia" (Johnson et al., 2000, p.67).  

REMARKS:  Amphibole is the group name for several related species and varieties, many of which have been given individual names -- e.g., tremolite The word amphibole was first applied to minerals of this group in 1801 by the famous French mineralogist Abbé René-Just Haüy (1743-1822).  His choice was based on the Greek word αμφιβολος (amphibolos  - equivocal or ambiguous) in allusion to  the diversity of composition and appearance of the species then known to comprise the group.  The rock name amphibolite is, of course, based on the mineral name.

The extraordinary appearance of the Wyoming rock depends at least in part on the fact that its amphibole grains have been partially to completely dissociated to a mixture of goethite and opaline silica.  This fact, which has been established by microscopic, x-ray, and chemical investigations (Dietrich et al., 1988), is not discernible macroscopically.

To date, I have seen ancient beads, but no other gemstones or ornaments, other than two handsome paperweights I have, fashioned from "run of the mill" amphibolites or amphibole gneisses. The amphibolite paperweight with its smooth, here and there glistening appearance makes me think these relatively common rocks have a good potential for use as gemrocks;  the second, a banded amphibole gneiss, is shown as Figure B in the GNEISS entry. 

SIMULANTS:  None that I have seen or seen described.

REFERENCES:  No general reference.

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Last update: 15 November 2011
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