This mineral is seldom even mentioned in the gem literature because it is so rare and has been so seldom cut. Faceted gems are practically nonexistent, and would be among the rarest of all cut stones.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
|Crystallography||Monoclinic. Crystals tabular, pseudorhombohedral; foliated, micaceous, radiating.|
|Colors||Oil green, olive green, yellowish green, brownish green, brownish.|
|Luster||Vitreous; pearly on cleavage.|
|Cleavage||Perfect and easy, 1 direction. Fracture uneven.|
|Stone Sizes||Very tiny green gems, less than 1-2 carats, have been cut from Connecticut material.|
|Formula||H2Na6(Mn,Fe,Ca,Mg)14(PO4)12 · H2O.|
|Pleochroism||Pale olive green to pale yellowish green.|
Optics: a= 1.648-1.658; β = 1.655-1.662; γ=1.662-1.671.
Biaxial (+), 2V ~90°.
Occurrence: A secondary phosphate mineral in granite pegmatites.
Branchville, Connecticut; Portland, Connecticut; Poland, Maine.
Comments: This mineral is seldom even mentioned in the gem literature because it is so rare and has been so seldom cut. Faceted gems are practically nonexistent, and would be among the rarest of all cut stones.
Name: After the Rev. William Dickinson in recognition of his interest in the locality where first found.