Sphalerite (ZnS), zinc sulfide, is a major zinc ore. Sphalerite has a resinous luster and a white to yellow and brown streak. Illinois sphalerite is generally yellow, yellowish brown, reddish brown, or brownish black. It is of medium weight (specific gravity is 3.9 to 4.1) and is brittle; it can scratch a penny but not a piece of window glass. Sphalerite is commonly opaque but may be translucent on thin edges.
Sphalerite was mined with galena in northwestern Illinois and in extreme southern Illinois with galena and fluorite. Small crystals occasionally are found in limestones, within geodes, and as crystalline masses in clay-ironstone concretions.