Agate is a common rock formation, consisting of chalcedony and quartz as its primary components, consisting of a wide variety of colors. Agates are primarily formed within volcanic and metamorphic rocks . Decorative uses of agates are known to date back to Ancient Greece and are used most commonly as decorations or jewelry.
Agate minerals have the tendency to form on or within pre-existing rocks, creating difficulties in accurately determining their time of formation. Their host rocks have been dated to have formed as early as the Archean Eon. Agates are most commonly found as nodules within the cavities of volcanic rocks. These cavities are formed from the gases trapped within the liquid volcanic material forming vesicles. Cavities are then filled in with silica-rich fluids from the volcanic material, layers are deposited on the walls of the cavity slowly working their way inwards. The first layer deposited on the cavity walls is commonly known as the priming layer. Variations in the character of the solution or in the conditions of deposition may cause a corresponding variation in the successive layers. These variations in layers result in bands of chalcedony , often alternate with layers of crystalline quartz forming banded agate. Hollow agates can also form due to the deposition of liquid-rich silica not penetrating deep enough to fill the cavity completely. Agate will form crystals within the reduced cavity, the apex of each crystal may point towards the center of the cavity.
The priming layer are often dark green, but can be modified by iron oxide resulting in a rust like appearance. Agate is a very durable and therefore is often seen detached from its eroding matrix, once removed, the outer surface is usually pitted and rough from filling the cavity of its former matrix. Agates have also been found in sedimentary rocks, normally in limestone or dolomite; these sedimentary rocks require cavities often from decomposed branches or other buried organic material. If silica-rich fluids are able to penetrate into these cavities agates can be formed.
I love that every item comes with the information cards explaining a bit of its history/qualities. Fun gift for my scientifically-inclined partner.