Topaz

Hardness: 8 (Mohs), Color: Colorless, pale blue, yellow, yellowish brown, orange, pinkish red; Luster: Vitreous; Crystal structure: Orthorhombic; Cleavage: Perfect on {001}; Specific gravity: 3.49 - 3.57.

Commonly occurs in fluorine-bearing, highly silicic igneous rocks like granite, granite pegmatite, and rhyolite. It also forms in cavities of volcanic rocks, hydrothermal veins, and greisens.

Predominantly used as a gemstone. Lesser quality or massive forms are uncommonly used as a source for aluminum.

Topaz is the state gem of Utah and the birthstone for November (yellow) or December (blue). Imperial topaz is highly prized.

Field identification often relies on its characteristic orthorhombic crystalline form, its high hardness of 8 (scratching quartz), perfect basal cleavage, and high specific gravity. Commonly found worldwide, major localities include Brazil, Russia, Sri Lanka, USA (Utah, Colorado), and Mexico. While found in Illinois, gem-grade crystals like this are rare and could be from elsewhere.