Among the various leisure activities that the state of Oregon boasts of, the agate hunting is one of the most thrilling and most enjoyable. While low on monetary value (exclusions exist), agates can be used to create stunning jewelry, beads, vases, ornaments, and other artworks. This type of chalcedony is considered to be semiprecious gemstones.
High-quality agates have distinctive bandings and color patterns, with "impurities" that make them unique and desirable. For example, "angel wing" - agate with a swirled grain is highly-prized. Other rare highly-valued agates: dendritic, sagenitic, moss, and plume.
Composed of silicon dioxide SiO2, agate is a translucent or semitransparent fine-grained cryptocrystalline quartz. The tough and attractive mineral has the excellent working properties, demonstrating a high hardness of 7 (the Mohs Scale), acid resistance, and retaining a polished surface quality. Agate consists of 90% quartz and 10% of moganite which has the same chemical composition SiO2 but a different crystal structure.
Generally, agate has distinctive colorful uneven bands or patterns. The color banding can be white, gray, orange, brown, pink, green, purple, black, yellow, and multicolored. Green and blue hues are rare. Embedded minerals, mostly iron oxides are responsible for color tones. Other inclusions are manganese, chromium, celadonite, nickel, copper, and hematite.
Agates (originally “Achates”) were discovered along the Sicilian Achates River (now Dirillo) in Italy between the 3d and 4th centuries BC by Greek biologist Theophrastus. He named the beautiful stones after the Achates River in which there were found. Since agate has very good physical properties, it has been used in many industries to burnish leather, crash and mix chemicals, create sculptures, paper knives, and seals.
The first gemstone workshop was recorded in the 14th century in the town of Idar-Oberstein, southwest of Germany. The industry used the local source of agate and amethyst until the 18th century when the local source was diminished. The gemstone production revived in the 1800s when German immigrants discovered great Agate deposits in Brazil. In the 20th century, Agate Idar-Oberstein industry grew up, thanks to new technologies in the chemistry, innovations, and new original artistic designs. Today Idar-Oberstein is the famous cutting, polishing, and trading center.
For thousands of years, people believed that agates have magical and mystical properties, providing protection, restoring energy and also used for religious ceremonies and diseases healing and prevention. From the astrology point, agate is one of the birthstones for people born under the Gemini sign (May 21 - June 20).
Agates form in many varieties of the host rock. Most of the agates we see today have their roots in ancient volcanic lava flows. Agates are secondary deposits that are housed in hollow gas cavities known as vesicles.
The formation of hollows and cavities can be traced back to the formation of the continents. Layers of molten lava containing trapped gases are forced towards the surface of the Earth. As the igneous rocks cooled and hardened, the gases escaped through cracks, leaving behind hollows. The empty cavities are gradually filled with fluid rich in suspended or dissolved minerals such as quartz (silica), moganite (quartz polymorph), calcite, or zeolites to form amygdules and geodes.
Super-saturation of silica creates an active crystallization front due to the formation of a gelatin-like consistency. As time progresses, tiny fibrous microcrystals attach themselves to the sides of the seam. The varying mineral impurities form contrasting bands. A balance between the silica and mineral impurities results in the formation of agates with alternating bands.
Agates are distinguished based on the type of formation or sometimes geographical location.
Blue Agate. The color is a result of mineralization within the earth during agate formation.
Carnelian (“flesh” in Latin) Agate. Colored by iron oxides, Carnelian Agate is a translucent reddish-brown or reddish–orange chalcedony.
Clear or White Agate. Milky agates are free of any impurities, typically semitransparent.
Dendritic (“tree-like” in Greek) Agate. The rarest agate has tree-like or fern-like patterns – mineral impurities locked inside as the agate was forming. Dendritic is not a real agate because it lacks bands. They are usually colorless, white, gray, and sometimes purple.
Enhydro (“water” in Greek) Agate or Water Agate. Agates with enclosed and movable around tiny bubbles of water or air that trapped inside millions of years ago. Enhydro agates may have other inclusions.
Eye Agate. Agate is formed when the majority of the silica gel is drained from the cavity but the remaining small droplets crystallize into solid concentric chalcedony known as "eyes". The other layers of silica arranged around the eyes.
Fortification Agate. Bands crystallize into concentric layers, where all bands connect to each other, creating the walls similar to the aerial view of a fort.
Geode Agate. As the supply of fluid rich in silica depletes, Geode Agate is formed with a hollow cavity. The cavity usually lined with small crystals such as quartz, zeolite, and calcite.
Iris Agate. Rare agate produces a unique iris effect. When the mineral is held towards a light, it shows prismatic colors of thin bands.
Moss Agate. The rarest translucent agate with embedded green mossy or fern-like inclusions. Due to Moss Agate lacks the banding pattern, it is not a true agate.
Plume Agate. The rarest agate contains plume-like patterns in a variety of colors.
Sagenitic (“net” in Greek) Agate. The rarest translucent or partially transparent chalcedony with fine hair or needle-like crystal inclusions.
Shadow Agate. Back and forth movement of agates causes a shadow effect to be formed as light entering into agates but is not reflected out.
Snakeskin Agate. It refers to a mineral with a layer that reminds the snake's skin. Also, reddish to brown agate containing small dark concentric bands.
Agate Hunting Locations
Agate has been found in many locations around the world including Australia, Brazil, Canada, Egypt, India, Indonesia, Madagascar, Mexico, Russia, Venezuela, and the United States of America (Arizona, Arkansas, California, Idaho, Oregon, Texas, and Washington).
Oregon is one of the most popular states for agate hunting. There are many public lands you can find gemstones for your collection.
Before accessing the area and collecting any rocks, contact the local agencies to find out the most up-to-date information and regulations.
2. "Oregon Rocks, Fossils Minerals". Josefine County. Oregon Historical Society.
3. Nancy Marie Brown. "How Do Agates Form?". 2001. Penn State University.
4. Dr. H.C.Dake. " The Gem Minerals of Oregon". 1938. Portland, Oregon. Oregon State Department of Geology and Mineral Industries.