Herderite is a rare collector gem, especially in larger sizes. It is too soft for wear but is attractive when cut. Clean stones are very hard to find. There is always the possibility of new and larger material coming on the market from Brazilian sources.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
|Crystallography||Monoclinic. Crystals stout or prismatic; tabular; crusts.|
|Colors||Colorless, pale yellow, greenish white, pink, green, violet.|
|Cleavage||Interrupted. Fracture conchoidal. Brittle.|
|Stone Sizes||Faceted gems from Maine are usually small (1-5 caratsl and pale colored or colorless. Brazil gems, however, have stronger color and may be up to 25-30 carats from larger crystals. Smithsonian Institution (Washington, D.C): 5.9 (green, Brazil). Devonian Group (Calgary, Alberta, Canada):3.65 (blue, Brazil). National Museums of Canada (Ottawa, Ontario): 4.65 (light violet octagon, Brazil).|
|Luminescence||Pale green in LW and SW; also pale violet in SW (Brazil, green gem) and stronger violet in LW. Orange fluorescence in X-rays with persistent phosphorescence.|
|Pleochroism||None or weak|
Optics: a = 1.591-1.594; β= 1.611-1.613; γ= 1.619-1.624.
Biaxial ( – ),2V= 67-75°. Can be (+). In general: herderite with (OH) is (+), with F is ( – ).
Note: Green gem from Brazil: a = 1.581; β= 1.601; γ= 1.610; birefringence 0.029; S.G. 3.02; contains 7% F.
Occurrence: Late stage hydrothermal mineral in granite pegmatites.
Germany; Austria: USSR.
Maine: colorless and pale yellow crystals.
Minas Gerais, Brazil: crystals up to nearly fist size, colorless and pink; some green, violetish.
Comments: Herderite is a rare collector gem, especially in larger sizes. It is too soft for wear but is attractive when cut. Clean stones are very hard to find. There is always the possibility of new and larger material coming on the market from Brazilian sources.
Name: After S. A. W. von Herder, a mining official in Freiburg, Germany.