Serpentine: Gemstones Information

There are two aggregate structures for serpentine(Latin-snake),leafy  antigorite (leafy serpentine) and fibrous chrysotile. Very finely fibrous varieties are called asbestos. It has a greasy to silky luster,and is sensitive to acids. Colors are often spotty. Minerals once classed as members of the serpentine group now findthemselves in the kaolinite–serpentine group which comprises hexagonal, trigonal, orthorhombic, monoclinic or triclinic silicates with the general composition M2–3 Z2O5(OH)4nH2O where M Al, Fe3, Fe2,Mg, Mn2, Ni, Zn; Z Al, Fe2, Si. Bowenite is antigorite as are the minerals for which the name ‘serpentine’ has long been in general nonspecific use.Kaolinite–serpentine group minerals are polished primarily for their attractive patterning though some may be intended to resemble the jade minerals nephrite and jadeite. Distinction is not too difficult by gemmological testing as both nephrite and jadeite have SG (3.0 and 3.3 respectively) higher than kaolinite–serpentine minerals. Using the validated names of today antigorite, clinochrysotile, and lizardite the SG and RI range for antigorite is 2.61, 1.560–1.571 respectively with birefringence 0.014. For clinochrysotile RI is 2.53–2.55 and 1.545–1.569, birefringence 0.001. For lizardite, RI is 1.546–1.560, birefringence 0.008.Nomenclature problems from which ‘bowenite’ may suffer also affect ‘williamsite’, also a variety of antigorite, occurring in notably translucentapple-green.Kaolinite–serpentine group minerals occur from the alteration of ultramafic rocks. Bowenite in translucent green colours is found in New Zealand; attractive green williamsite is found at Rock Springs, Maryland,USA. Banded material is found in many places.