Introduction to Agates

What is an agate?

agate banding
Agate is a sedimentary silica-rich rock composed of the mineral "chalcedony," which is a term generally referring to the fibrous growth of quartz; as well as more common forms of quartz. It is commonly identified by its sequenced or "banded" layers which display a variable amount of patterns and colors based on deposition type and influence by the host rock mineralogy, respectively. It is a very hard rock, and has been used by mankind throughout recorded history for jewelry, weaponry, and more recently as a chemical additive for adjusting acidity (pH).

How is agate formed?
Although agates are hosted by a number of different rock types, the primary host is volcanic rocks such as lava flow deposits like basalt. As volatile gases escape the molten lava during its surface deposition and cooling, gaps called vesicules are left within the hardened rock. It is within these voids that secondary mineralization occurs, typically with the help of fluid migration; although its formation can be influenced by heat and pressure as well.

As groundwater and percolating fluids enter individual vesicles, over time they precipitate (to render solids from solution) silicate minerals within the voids. Each layer within an agate starts out as an ooze, but over time hardens as it combines to form crystals. Using a microscope, one can see the crystals within agates develop in a fibrous pattern with fiber growth occuring nearly perpindicular to the growth surface. However, to the bare eye, the banding and layers are sometimes worthy of awe.

crystal growth in agate voids

At times voids are completely filled with progressive layers of mineral crystalizations, other times only a portion of the void is filled, leaving a gap that can experience additional mineral development at later stage(s). Quartz crystals or druse often grows within the remaining void space adjacent to banded layers of chalcedony and quartz. The picture above shows two mineral growths that occurred after the fortification agate deposit terminated (black and translucent minerals at center).

Agates are usually found as nodules within mafic volcanic matrices. The outside of the nodule is usually influenced by iron and appears reddish brown and rusty, and is typically the mineral limonite.

What are the different types of agates?
The list of names for agates is long and is usually chosen based on the growth structure, the rock in which the agate is found (host rock), or is identified by a unique combination of colors and patterns.

agatized wood

YupRocks has picked a few variations of agates for discussion via the links provided below. Each page contains a photo gallery (high resolution) to provide examples of the type and help solidify concepts.

Fortification Agates
Water Line Agates
Agates in Petrified Wood
Thunderegg Agates

Additional Learning
The Quartz Page - fantastic and detailed discussion of the formation of agates. Microscope pictures and cross sections of agates showing crystal growth and patterns!
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