The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
Highest values go to faceted gems, particularly Hexagonite and chrome Tremolite, and strong cats eyes.
|Colors||White, colorless, gray, pale greenish, pink, brown.|
|Wearability Notes||Good (aggregates); Poor (crystals)|
|Cleavage||Good 2 directions|
|Stone Sizes||Small colorless and transparent tremolite crystals are very rare, and cut gems are true collector items. The largest of these is in the 5-10 carat range. Larger crystals exist but are usually badly fractured. Hexagonite is known in facetable material only from New York and these pieces yield gems to only about carat. Chrome tremolite is also very rare and cut gems are tiny. Catseye hexagonites have also been cut. Private Collection: 1.21 (medium purple, New York). National Museums of Canada (Ottawa, Ontario): 1.39 (deep purple, New York); 4.55 (dark blue catseye, Ontario); 12.55 (dark brown catseye, Ontario).|
|Formula||Ca3Mg5Si8O22(OH)2 + Fe.|
|Crystallography||Monoclinic. Crystals prismatic or bladed; fibrous, massive, granular.|
|Birefringence||0.017-0.027; hexagonite 0.019-0.028.|
|Luminescence||Tremolite not diagnostic. Actinolite inert. Hexagonite shows orange, medium pink to pinkish red fluorescence in LW, SW. Also, medium greenish white in SW (Lee, Massachusetts) and dull yellowish in LW.|
|Luminescence Type||Fluorescent, UV-Long, UV-Short|
|Absorption Spectrum||Not diagnostic. Actinolite: faint line at 5030. Some tremolite shows a line at 4370 typical of jadeite. Chromiferous material may display chromium spectrum.|
|Pleochroism||Hexagonite: bluish-red/deep rose/deep red-violet. Tanzanian green crystals: light yellowish green/light green/green.|
|Optics||a = 1.560-1.562; β = 1.613; γ = 1.624-1.643. Biaxial (-), 2V= 81°. Note: Tanzania, green crystals: 1.608-1.631.|
|Optic Sign||Biaxial -|
|Fracture Luster||Vitreous to dull.|
|Specific Gravity||2.9-3.2 (catseye gem, Ontario, 2.98; hexagonite, 2.98-3.03).|
|Transparency||Transparent to opaque, actinolite usually translucent.|
a =1.560-1.562; β= 1.613; γ=1-624-1.643
Biaxial (-), 2V= 81°.
Note: Tanzania, green crystals: 1.608-1.631, S.G. 3.02.
Tremolite occurs in contact and regionally metamorphosed dolomites, in magnesian limestones, and in ultrabasic rocks.
It is possible to misidentify tremolite, mistaking it for other amphiboles. Hexagonite is the rarest of the gem varieties of tremolite. If tremolite occurs in very tiny fibrous crystals, densely matted and interlocked, it is then known as nephrite (jade). Material containing more or less parallel fibers is somewhat chatoyant and yields weak catseyes. These are sometimes called catseye jades, but have been tested and are actually tremolite or (if more iron-rich) actinolite.
From the Tremola Valley on the south side of St. Gotthard, Switzerland. Hexagonite was so named because it was thought to be a hexagonal mineral when first described.
Hexagonite, purple Tremolite