|Transparency||Transparent to opaque|
|Luster||Vitreous to resinous|
|Fracture||Conchoidal to uneven|
|Optical Character||Uniaxial+ Double Refractive|
|Refractive index||1.691 - 1.719|
|Pleochroism||Strong; yellow green|
|Comments||attacked by hydrochloric acid|
Willemite (WILL-ah-mite), a silicate of zinc, bearing the formula Zn2SiO4, is another rare mineral that is occasionally cut for collectors when pieces of sufficient transparency are available. Greenish yellow is the most commonly encountered color, although green, red and brown crystals are also sometimes found. The mineral was named in 1830 after William I, King of the Netherlands.
The willemite that is suitable for cutting occurs in hexagonal, prismatic crystals and is usually strongly fluorescent. It is also found in small grains and granular masses. Its properties are as follows: hardness, 5 1/2; toughness poor; cleavage one easy direction parallel to the basal face of the rhombohedron; fracture conchoidal to uneven; S.G. 3.90 to 4.10; streak white; characteristic inclusions none; luster vitreous to resins; R.I. 1.691 - 1.719; optic character uniaxial positive; heat fuses with difficulty; acids attacked by hydrochloric.
Willemite forms in zinc-ore deposits and is associated commonly with calcite, franklinite, zincite and rhodonite. Gem quality material is found only at Franklin, Sussex County, New Jersey. Other sources Include Altenberg, Belgium; Musartut, Greenland; Mindouli, Republic of the Congo-Brazzaville; Northern Rhodesia; South-West Africa; Socorro County, New Mexico; and Clifton, Arizona.
Little difficulty is encountered in cutting and polishing willemite, except that the facet junctions sometimes tend to crumble. Attractive stones can be made by polishing on a tin lap with Linde A powder and using crown and pavilion angles of 37° and 42°, respectively.