Chalcedony

Chalcedony is a variety of quartz.
Formula: SiO2 tectosilicate (framework silicate)
Specific gravity: 2.6
Hardness: 6½; to 7
Streak: White
Colour: Colourless, white, gray, blue, any colour due to embedded minerals; multicoloured specimens not uncommon.
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Volcanic igneous environments
Sedimentary environments
Metamorphic environments
Hydrothermal 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.