Agate, Mexico, “Fortification Agate” (a picture agate with a pattern that resembles a fortress), Slab ~ 4 inches across. Photo ? Joel E. Arem, PhD, FGA. Used with permission.
Agate is a variety of chalcedony, a cryptocrystalline form of quartz. Translucency, patterns of color, or moss-like inclusions may distinguish this stone from other forms of chalcedony. Agates can show a wide variety of vivid, multiple colors. These are principally the result of traces of oxides of iron, manganese, titanium, chromium, nickel, and other elements. All agates take a wonderful polish and are tough enough for most jewelry uses. Designers often take advantage of the intriguing patterns these stones have to offer to create unique and?fascinating?pieces.
In general, agate values are quite modest. Their prices reflect mainly labor and artistry rather than the value of the material itself. Agates of large size or with particularly distinctive, fine, or landscape-like color patterns are at a premium. Custom cut pieces or stones from collectible locations would be substantially more expensive.
From the Ancient Greek Achates, the name of a river in southwestern Sicily where the material was found.
Varieties of Agate
The color patterns in agates usually take the form of flat or concentric layers or bands. Mossy or dendritic inclusions can sometimes create the impression of vegetation and landscapes. ?Varieties of this gemstone are described by their color patterns, inclusions, or source.
With regular color layers and bright colors, this variety is one of the most popular. These stones are found all around the world, but Brazil is one of the most productive sources. Many of the richly colored, banded agates you see for sale are dyed.
These stones contain mossy inclusions of mineral oxides that may be any color. Stones that present plant-like patterns are called moss agates. Those with feather-like patterns are called plume agates. Those with tree-like, branching patterns are called dendritic agates.
Fossilized ancient tree trunks and limbs may have their organic components replaced by agate stone over millions of years. In some cases, their woody structure may?also be preserved and?visible?with a microscope. The agate’s color may be very bright and strong.
Dyeing is an ancient and common practice for enhancing agates. (Chalcedony stones are relatively porous). This is usually a stable process.
A celebrated secret process for dyeing agates was developed in Idar-Oberstein, Germany, in the 19th century. When the agate deposits in this town were depleted, emigrants to South America shipped grey agates back to Idar-Oberstein. These pieces were dyed and the results were extraordinary.