Calendar of Events
Summer 2009

About Agates
By Michael R. Carlson

 Agates are among the most marvelous and varied works of nature. Due to their availability, beauty

and durability, they have been collected and worked since time immemorial. Gathered and kept as treasured objects, agates have long been used and fashioned into such items as jewelry, charms, seals, carvings, bowls, weapons, tools, bookends, marbles and so on.
    Agates exhibit an array of patterns and colors that no other gemstone can match. Unlike other gems, each individual agate is unique in color and design. Agates are cuttable, yet tough, and take a very high and lasting polish. They are recognized by the general public, but also studied by scholars. Agates can be seen in the finest museums, but are found as well on the shelves of amateur collectors.
    The world’s agates are widely distributed, and can be found in igneous, sedimentary, and metamorphic areas. They have been collected from deserts, mountains, rivers, and ocean shores. The word “agate” itself is likely derived from the ancient Achates River in Sicily, where the first written description of agates was found.
    The basic material of agate (ag?it) is chalcedony (kal sed?’n e), which is silicon dioxide (SiO2) in the form of microscopic fibrous crystals, called cryptocrystalline. While pure chalcedony is typically colorless to white, agates are a variegated form of chalcedony, which can range from translucent to nearly opaque. Metallic salts or oxides primarily color them, with red, brown, yellow, and gray the most common shades. Essentially, agate is a colored variety of chalcedony, which can exhibit bands, dots, clouds, or moss-like formations.
 

   Other physical characteristics of agates include: a glassy or waxy appearance, no cleavage direction, a shell-like conchoidal fracture, and a specific gravity near 2.60. The hardness of agates varies around 7 on the Mohs scale. Agates are somewhat porous, which allows them to be dyed with various chemicals and acids. Fluorescence of agates is rare, and is usually greenish if it occurs. Although many agates are found as nodular varieties, agates can also be found in other forms such as veins, or replacements of wood and coral.
    There are several differing theories of banded agate formation, but no single theory has yet accounted for all of the varieties and structures exhibited. Generally, however, there must have been a deposition of silica into vesicles, or chambers, formed in surrounding rock. Ground waters possibly dissolved the silica from the host rock in which the agate was formed. This silica was likely to have been gelatinous for a period of time. Because agates are more resistant than the surrounding rock, they are usually released over time by weathering processes.

Agate Quality
    While beauty may indeed lie in the eye of the beholder, there are certain features that help top agates stand apart from the rest. These observable traits can be divided into several noteworthy categories including pattern, color, shape, size, and rarity. Other defined unique characteristics can also influence perceived quality.
 

   Pattern refers to the overall design of the banding. The pattern may be regular and concentric, or it may be more free form in nature. The best stones are usually the ones with a strong pattern and contrasting colors. Whole and complete patterns are much preferred over broken ones.
The color of an agate will greatly affect its overall quality. The coloring may be bright, subtle, or a combination of each. Agates showing several strong colors are often the most desirable, especially if these colors are intense or unusual.
The shape of the agate can add or detract from its appeal. If the shape is not pleasing as a whole, then the effects of banding, color, and other features will be diminished. Complete shapes are far more preferable than broken ones. Shape gives definition and presence to the stone.
    Size may not be as relevant to a stone’s quality as other attributes, but top agates of large size can be eye catchers. Maximum size does vary among the many varieties of agates. Queensland agates, for example, are generally quite small, with most being under one-half pound. Brazilian agates, on the other hand, are quite often several pounds in size.
    Solidity (lack of fractures, crystalline quartz, and matrix) also helps determine the quality of the agate. All varieties of agates are to some degree affected by fractures or cracks. There are many different causes of this fracturing. Regardless of the reason, cracks can impair stones of otherwise top quality.
All agates are a variety of quartz, but the amount of crystalline quartz seen can also affect their appearance. Generally less visible quartz of this type is preferred, although it may occasionally enhance the look of an agate. For example, quartz geode centers can add an extra sparkle to some stones.
 

   Matrix can also detract from the general appearance of an agate. Agates that show large amounts of their host matrix material are usually not as appealing as specimens that are composed entirely of agate. An exception might be an agate nodule still largely enclosed in the matrix in which it was formed. This type of specimen can yield valuable insight into the formation process.
    Choice combinations of top pattern, bright colors, good shape, large size, solidity, and unique characteristics are indeed rare. Most exceptional agates include several or all of these features.

    This article was excerpted from The Beauty of Banded Agates An Exploration of Agates from Eight Major Worldwide Sites by Michael R. Carlson. The book is available in the Museum Gift Shop and online at www.agate-nodule.com. Many thanks to Mike Carlson, Austen Cargill and Doug Moore for their help in putting together the special exhibit “Worldwide Agates.”