Chalcedony (Silicate-Framework) 

Si02

Hardness: 7.0
Specific Gravity: 2.65

Characteristics: Fibrous microcrystalline variety of quartz; vitreous luster; conchoidal fracture; only soluble in HF.

A common mineral found in igneous rocks, many metamorphic rocks, and most sedimentary rocks.

Chalcedony is primarily used as a decorative stone in collections and some jewelry.

(Primary text: Perkins, D. 2002. Mineralogy. Prentice Hall.)