Pennsylvania State Gemstone
Williamsite is a translucent to nearly transparent variety of serpentine and a synonym of the mineral antigorite. It occurs in association with magnetite, chrysotile, olivine, and chromite. Chromite is the mineral responsible for williamsite’s color. A magnesium silicate mineral, williamsite is light green to emerald green, blue-green, or white with black crystals often included. Chrysotile and lizardite are closely related members of the serpentine group.
Williamsite was first discovered at Woods Chrome, Pennsylvania, in 1849. Today, Line Pit, a chromite mine on the Pennsylvania-Maryland state line, is known for the highest quality williamsite. Mining of chromite, an associate of williamsite and the mineral responsible its color, was an important industry in Pennsylvania in the 19th century.
Although williamsite is actually named for Lewis Williams, who first described it, the name calls to mind Pennsylvania’s founder, William Penn. So although Pennsylvania does not have an official state gemstone, williamsite is representative of the state’s geology and history. Lewis W. Williams also first described zaratite (emerald nickel) the same year he described williamsite.
Lewis Williams first discovered williamsite at Woods Chrome, Pennsylvania. Antigorite was named for the occurrence at Val Antigorio, Piedmont, Italy.
FORMATION AND OCCURRENCE
Serpentinite is a major rock-forming mineral. It is found in many igneous and metamorphic rocks, and may be responsible for the color of any green rock. Williamsite commonly replaces rocks that contain iron or magnesium but little or no silica. It forms microscopic masses.
Williamsite is fairly widespread. The following is a list of some localities with well-studied material:
Williamsite is cut into cabochons and beads for use in jewelry.
Chemical Formula: (Mg;Fe2+)3Si2O5(OH)4 + or - nH2O
Crystal Structure: Monoclinic
Hardness (Mohs): 2.5
Color: Yellow, green, brown, black, cream-white
Transparency: Translucent to opaque. Rarely transparent.
Luster: Greasy, waxy, or silky
Refractive Index: 1.560-1.571
Cleavage: Usually not discernible because of crystal development. Chrysotile may exhibit basal cleavage.
Fracture: Conchoidal, splintery
Some data courtesy of the Mineralogical Society of America
Author: World Trade Press