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Agate Gemstones

Author: , Ph.D., GIA Graduate Gemologist

Montana agate collection

Montana agate: A bright collection of agate cabochons cut from rough found in Montana. They show a diversity of banding patterns and inclusions. Image copyright Geology.com.

What is Agate?

Agate is a translucent variety of microcrystalline quartz. It is used as a semiprecious stone when it is of desirable quality and color. Agate generally forms by the deposition of silica from groundwater in the cavities of igneous rocks. The agate deposits in concentric layers around the walls of the cavity, or in horizontal layers building up from the bottom of the cavity. These structures produce the banded patterns that are characteristic of many agates.

Agate occurs in a wide range of colors, which include brown, white, red, gray, pink, black, and yellow. The colors are caused by impurities and occur as alternating bands within the agate. The different colors were produced as groundwaters of different compositions seeped into the cavity. The banding within a cavity is a record of water chemistry change. This banding gives many agates the interesting colors and patterns that make it a popular gemstone.

Polished agate slab

Agate slab: A polished agate slab cut from a nodule with an interesting history. The nodule was first infilled by horizontal layering of silica, then concentric infilling, and finally by horizontal filling. Image copyright Geology.com.

Crazy Lace agate

Crazy Lace cabochon: A cabochon cut from Mexico's Crazy Lace agate. Crazy Lace has an infinite variety of bands, orbicular structures, and lacy patterns. Image copyright Geology.com.

Agate Gemstones:

Agates have been used as gemstones for thousands of years. They were some of the earliest stones fashioned by people. Today they are cut into cabochons, beads, small sculptures, and functional objects such as paperweights and bookends. Agate cabochons are popular and used in rings, earrings, pendants, and other jewelry objects. Agate beads are commonly made into necklaces and earrings. Some have been used as marbles.

Coyamito agate

Coyamito agate: A round cabochon cut from Coyamito agate. Image copyright Geology.com.

Agate bookends

Agate bookends: A matched pair of bookends cut from a large agate nodule. The blue color was produced with dye. Image copyright Geology.com.

Agua Nueva agate

Agua Nueva agate: A colorful cabochon cut from Agua Nueva agate. Image copyright Geology.com.

Tumbled Agate:

Agate is the most popular rough for producing tumbled stones. It is generally inexpensive and can be tumbled with good results by beginners. It has a hardness of seven and can be loaded into a rock tumbler with jasper and any of the quartz varieties.

Petrified wood agate

Petrified wood: Cabochons made from petrified wood. Image copyright Geology.com.

Landscape agate

Landscape agate: A polished slab of landscape agate that yields a "Monument Valley" scene. Interesting landscape agates are prized by collectors. Image. copyright Geology.com.

 Rocks and Minerals Commonly Tumbled
Lapis Lazuli
Petrified Wood
Rose Quartz
Smoky Quartz

More About Agate:

Most agate has unimpressive colors and patterns. However, agate is a porous material that readily accepts dye. Most of the spectacularly colored agates sold in the gemstone trade have been dyed. Rarely, the color patterns of an agate form interesting landscape scenes. These are sought after by collectors.

Orange crack agate beads

Crack agate beads: Barrel-shaped beads cut from orange crack agate. Image copyright Geology.com.

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