Agate is a variegated class of Chalcedony, usually displaying different color bands or other patterns- though some Agate is not patterned and displays opaque translucency. Agate was first discovered by Pliny in 77 A.D. and was named for the River Achates in Sicily.
Chemical composition: SiO₂ with other polymorphs of silica; Hardness: 7
Although Opal is still considered a mineral for historical reasons it is actually not a true mineral. It is amorphous, having no definite chemical composition or chemical structure, and is therefore referred to as a “mineraloid.” The composition of Opal is numerous tiny silica spheres stacked and bonded with silica and water called hydrated silicon dioxide. Once solid, Opal maintains a water content of 3%-21%.
Chemical composition: SiO2 · nH2O ; Hardness: 5.5-6.5
Silver was known as Argentum in ancient Rome, thus its abbreviation as Ag. It is a member of the copper group.
Chemical composition: Ag; Hardness: 2.5-3