Lets Talk Gemstones

By Edna B. Anthony, Gemologist
(Contact the author for permission to reproduce this article in any form.)
P.O.# 62653; COLORADO SPRINGS, CO. 80962




One year during a visit to the famed Tucson show, the author discovered a startlingly beautiful golden-yellow faceted gemstone identified as clinohumite, purported to be from the Lake Baikal region in Siberia. The dealer could provide little information about the gem’s physical and optical properties. Thus began a search to determine how practical its use for jewelry might be.
The Mineralogical Record, Inc. publication Glossary of Mineral Species lists alleghanyite, chondrodite, clinohumite, humite, jerrygibbsite, leucophoenicite, manganhumite, norbergite, rubellite and sonolite as members of the humite group. Walter Schumann mentions none of these in his Gemstones of the World. Of the ten species, only chondrodite, clinohumite and norbergite are listed in Michael O’Donoghue’s American Nature Guides Rocks and Minerals. Dr. Joel Arem adds humite to the list in his Color Encyclopedia of Gemstones. The discussion of the humite group in Cornelius S. Hurlbut, Jr. and Cornelis Klein’s Manual of Mineralogy after J. D. Dana tells us the members of the group are so similar in chemistry, structure, and physical characteristics that optical tests must be used to distinguish one from another. A structure that alternates layers composed of Mg(OH,F)2 with others, which possess the atomic arrangement of olivine, create a close relationship to the structure of olivine. Hydroxyl end members are not known to occur, although the replacement of iron by OH can be extensive. Each specie is biaxial positive and exhibits a vitreous luster. Poor cleavage and infusibility are common to all. Exposure to long-wave ultra-violet light reveals a dull orange luminescence. While most specimens remain inert under short-wave light, the exceptions exhibit a bright yellow fluorescence. TiO2 in humite and clinohumite greatly influences their optical properties.

All of the species develop in contact regions of metamorphic metasomatised limestone and dolomites and in skarn derived from such deposits, into which large amounts of magnesium, iron, aluminum and silicon have been introduced. They are seldom found in alkaline formations of igneous origin. Dr. Arem mentions a source in a carbonatite formation in Loolekop, East Transvaal in Africa, but he does not specify if the formation is of igneous magmatic or sedimentary origin. The American Geological Institute’s Glossary of Geology states that although “carbonatite” has been used synonymously with “limestone,” the more common use of the term designates an igneous rock. Wiberforce, Ontario, Canada is a source of crystals of the humite group, and deposits in Pargas, Finland and Kafveltorp, Orebro, Sweden produce yellow material. Noted sources of the specific species are listed in the narratives below. The characteristic small crystals of the group often harbor numerous inclusions and fractures, so faceted gems larger than three carats are extremely rare.


This most common member of the humite group [Mg5(SiO4)2(F,OH)2] usually occurs in granular and massive forms. Its tiny transparent to translucent yellow, red and brown monoclinic crystals frequently show lamellar twinning. The Manual of Mineralogy tells us its association with graphite, phlogopite, pyrrhotite and spinel in crystalline limestone formations is highly characteristic. It occurs with fosterite, monticellite and wollastonite in skarn deposits. The Tilly Foster magnetite deposit near Brewster, New York yields most of the available gem crystals, although it is also found in Kafveltorp, Sweden, the Pargas area in Finland and Monte Somma, Italy. The density can vary from 3.16 – 3.26. It possesses a hardness of 6.5. Refractive readings range from 1.592 to 1.646 with a birefringence variation from 0.028 to 0.034. Dr. Arem lists pleochroism colors of very pale yellow/brownish yellow – colorless/yellowish green – colorless/pale green. The combination of toughness and good hardness with rich colors would make chondrodite a lovely gemstone, but the lack of size makes the few faceted gems prized additions to a collector’s cabinet.


The granular form and the frequently highly modified yellowish to orange and brown transparent to translucent crystals of norbergite [Mg3(SiO4)(F,OH)2] develop in the orthorhombic crystal system in contact regions in dolomite and limestone formations. The name reflects the source located in the Ostanmosoa iron mine in Norberg, Sweden. The Franklin area of New Jersey also produces this specie of humite. Scratch tests reveal a hardness of 6.5 on the Mohs scale. With a range of 3.15 to 3.18, it has the lowest and least variable density of the four gem humites. Its refractive indices are also the lowest, with readings of 1.563 to 1.593 and a birefringence of 0.026 to 0.027. The dichroscope reveals the three pleochroism colors of pale yellow, very pale yellow, and colorless. Like chondrodite and humite, the crystals are so small that the very few gemstones cut from this material are the province of collectors.


Orthorhombic humite forms a series with monoclinic clinohumite. Its chemical formula can be written as Mg7(SiO4)3(F,OH)2. Gemmy, small yellow and intense orange translucent to transparent crystals are found in association with clinohumite and chondrodite at the Tilly Foster Mine. Humite exhibits a hardness of 6 on the Mohs scale, a density of 3.20 to 3.32, and refractive index readings from 1.607 to 1.675. The birefringence varies from 0.029 to 0.031. Trichroism colors of yellow - colorless/pale yellow - colorless/pale yellow are seen with the dichroscope. Suitable rough for cutting is extremely small and almost unobtainable, so faceted humite is almost unknown. These tiny gems would certainly be collectors’ items.


Monoclinic clinohumite [Mg9(SiO4)4(F,OH)4] is an end member of the humite to clinohumite series The yellow, brown, orange, and white transparent to translucent crystals can be found in contact zones in dolomite formations, talc schists, and in serpentines. The Tilly Foster iron mine, the Pargas area in Finland, and the Llanos de Junar, Malaga, Spain are sources of this specie, but the largest and finest intense yellow and orange crystals are recovered in the Pamir Mountains and the Lake Baikal region of Siberia. At refractive indices of 1.631 to 1.668 and a birefringence of 0.037, the optics of the material from Siberia vary from the normal refractive readings of 1.629 to 1.674, with a birefringence range of 0.028 to 0.041. When viewed through the dichroscope, clinohumite’s pleochroism colors of golden yellow/deep reddish yellow - pale yellow/orange yellow and pale yellow/orange yellow are revealed. A variation in density can range from 3.17 to 3.35, with a hardness of 6.

Gems cut from clinohumite are not common, but they can be larger and more abundant than gems of its sister species. They possess excellent toughness, a fair hardness, and bright intense colors. For those who appreciate the unique, one set in an appropriate ring mount and treated with care or a pair featured in earrings would be an asset to one’s collection of jewelry.