Chalcedony is a "variety" of quartz, the most common silicate mineral in Earth's crust. Quartz is ~pure silica - SiO2. It has a nonmetallic luster, can be any color, has a white streak, a hardness of 7 on the Mohs Hardness Scale, forms hexagonal crystals, has no cleavage, and breaks with conchoidal fracture.
Chalcedony is finely-crystalline, translucent, fibrous quartz. It often forms irregularly concentric layers. Many geodes are lined with chalcedony, on top of which are usually large quartz crystals. Agate is another "variety" of quartz - it consists of chalcedony.
The specimen shown here is an attractive blue chalcedony from near Kemmerer, Wyoming. It has grown on the walls of a cavity in fossil wood. The wood itself (see next picture in the photostream) is quartz-permineralized ("petrified wood") and was deposited in ancient Lake Gosiute, a large inland body of water that covered much of southern and southwestern Wyoming, plus parts of far-northwestern Colorado during the Eocene. Blue chalcedony-bearing fossil wood from this area is nicknamed "Blue Forest Wood".
Locality: unrecorded/undisclosed site near Fontenelle Reservoir, northeast of the town of Kemmerer, southwestern Wyoming, USA
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