Chalcedony is a variety of quartz.
Formula: SiO2 tectosilicate (framework silicate)
Specific gravity: 2.6
Hardness: 6½； to 7
Colour: Colourless, white, gray, blue, any colour due to embedded minerals; multicoloured specimens not uncommon.
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Volcanic igneous environments
Chalcedony is deposited from aqueous solutions, and is frequently found lining or filling cavities in volcanic rocks, as crusts in epithermal (low temperature) to mesothermal (medium temperature) hydrothermal veins, as a constituent of silica-rich marine sedimentary rocks, and as nodular concretions and layers in limestone and marl.