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The term “river rock” describes an array of rocks that have been worn smooth by the action of moving water. They can be sedimentary, igneous, or metamorphic, depending on the geography of the river in which the rocks occur. Rivers that flow through volcanic areas will have igneous rocks such as basalt and granite. Sedimentary rock such as limestone, dolomite, and siltstone is commonly formed in or near water. Metamorphic rocks, like marble and schist, are also found in rivers.
Granite, basalt, schist, limestone, shale, Jasper, agates, white or pink feldspar, and clear quartz are commonly found in rivers. Petrified wood is also found in rivers or sticking out of riverbanks. A river continuously erodes its banks, freeing many types of rock for later discovery downstream.
Depending on their weight, rocks, and stones that enter the river can be transported many miles downstream and deposited in shallower areas on gravel bars. Sediment is a solid material that is moved and deposited in a different location by water and consists of rocks, minerals, and plant matter. It is often deposited in the river delta and at the bottom of waterfalls.
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Most Common Rocks You Can Find in Rivers
Rivers are often great places for rockhounding. You can find there a lot of beautiful rare rocks. Let’s take a look and the ten most common rocks you can find in rivers in the sections below.
Granite is a common type of rock found on the banks of rivers and in river beds. It is an igneous rock that forms from lava flows and often contains amphibole, quartz, or mica.
Granite varies widely in color from purple to white, black, gray, and green and is the most familiar kind of igneous rock to many people.
Basalt is typically dark grey or black due to high augite or pyroxene content and has a range of shading. It is an igneous rock formed from ancient lava flows.
The Columbia River Basalts consists of seven formations: Steens, Imnaha, Grande Ronde, Picture Gorge, Prineville, Wanapum, and Saddle Mountains. It is found mainly in eastern Oregon and Washington, western Idaho, and northern Nevada.
Some basalts contain gemstones like corundum, zircon, and garnets. Raw Oregon Sunstones are found in basalt.
Schist is any medium to coarse-grained metamorphic rock formed from shale or mudstone and is often interleaved with feldspar and quartz.
It is usually named for other rocks found within it, such as garnet schist, tourmaline schist, or mica schist. It is typically gray but can have a banded, ribbed, or striped appearance. Schist is a host rock for several types of gemstones such as sapphire, kyanite, garnet, chrysoberyl ruby, and iolite.
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- Smithsonian Handbooks: Rocks & Minerals
- Gemstone & Crystal Properties (Quick Study Home)
- Ultimate Explorer Field Guide: Rocks and Minerals (National Geographic Kids)
Most limestone is due to marine deposits, but it also forms in rivers and lakes on land. Rivers flowing through limestone or underneath it often create spectacular caves and rock formations.
It consists of more than 50% calcium carbonate from the remains of countless marine animals that lived millions of years ago and is easily eroded by water.
Limestone is popular with fossil hunters because it contains so many fossil deposits. It can also have larger crystals of calcite and calcite-filled geodes.
Calcite can be clear, white, gray, yellow, blue, orange, violet, brown, or even black. The river that flows through the Blue Spring Caverns in Indiana is the longest underground river in the United States.
Shale is a fine-grained sedimentary rock that forms from silt, and clay-sized mineral particles commonly referred to as mud. It consists of many thin layers and splits easily into thin pieces along the laminations.
The color of shale depends on the presence of specific materials such as iron or organic substances. It is often black and grey but can be red-brown or yellow if deposited in oxygen-rich environments containing iron.
Shale is typically deposited in slow-moving water and is usually deposited in river deltas or flood plains.
Ordinary Jasper is a dense opaque rock with a uniform red color. However, it can occur in yellow, brown, or dark green.
Its appeal lies in its attractive patterns and color formations Morrisonite is a multicolored jasper from the Owyhee River gorge in Oregon. Jasper can be found in the Nehalem River’s gravel bars and the riverbed of the Clackamas River in Oregon.
A variety of gem-quality Jaspers may be found on the gravel beds along the Willamette River banks from Portland to Eugene. Large boulders of Jasper in various colors can be found in Eel River at Hearst and north of Potter Valley, both in Mendocino County in California.
TIP: California is an amazing place for rockhounding. You have a lot of options for rockhounding in this state. Check out the best places to dig for gems in California in the article below:
There are many different kinds of agate, a form of chalcedony, characterized by a waxy luster and banding. Carnelian agate is the name given to chalcedony that is a translucent orange-red or brown. Rockhounds around America prize Montana agate for the beauty of its designs and its hardness.
When river hunting for agates, look for areas where the water has exposed low gravel bars. Lake Superior agates have been found in deposits in the Mississippi River.
These agates were ripped from billion-year-old lava flows in Lake Superior’s bed by glaciers and traveled down the river to Natchez. Lake Superior agate is well known for its rich red, orange, and yellow coloring.
A five-acre area along the Calapooia River in Linn County, Oregon, is famous for a stunning lavender agate known as Holly Blue. Montana agates occur along the entire length of the Yellowstone River.
Montana Moss agate is a transparent to translucent chalcedony with a white, grey, yellowish, or reddish-brown base color and gets its name from black dendritic or mossy inclusions.
TIP: Agates are popular among all rockhounds and Oregon is one of the best places where to find them. Check out the complete guide about rockhounding in Oregon where you find everything you need to know:
Feldspar is the name of a group of closely related anhydrous minerals containing aluminum and silica and one or more basic elements such as potassium, sodium, or calcium.
It is often found in igneous, metamorphic, and sedimentary rock in rivers. Labradorite is a lovely iridescent feldspar that shimmers with colors of blue, green, red, orange, and yellow. This is called labradorescence.
Gem quality feldspar is colorless and glass-clear, but these are rare. Pink, grey, and white are much more common. “Sunstone” is the name of a type of feldspar that has a brilliant pink to reddish metallic glitter or shimmer.
Potassium feldspar and calcium and sodium feldspars produce gem-quality stones. Amazonite, moonstone, and orthoclase are well-known feldspar gems.
When quartz-bearing rocks are eroded, quartz grains are concentrated in the soil and rivers. White sand typically found in riverbeds is mainly composed of quartz.
Unusual blue quartz is located in Virginia and has a pearly or waxy luster. The color of blue quartz ranges from light sky blue to dark grayish-blue. Quartz comes in many different colors, such as pink (rose quartz), purple (amethyst), clear, white, yellow, black, green, and red.
Gravels along the Robinson River in Madison County contain blue quartz. In Greene County, blue quartz can be found in the stream gravel of Middle River, while in Amherst County, it is in the stream gravel beds of the Pedlar and Buffalo Rivers.
The Tye River in Nelson County also has blue quartz with white quartz. In Texas, blue quartz can be found in the Llano River together with many other types of rocks like clear quartz, topaz, agate, fluorite, garnet, and smoky quartz.
Petrified wood comes in different kinds, for example, silicified wood in which the wood has been replaced by silica or opalized wood where the wood has been replaced by opal. In both cases, the original wood grain is usually preserved and clearly visible in the rock.
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Tips For Finding Rocks in Rivers
A river grinds at the bedrock when it flows over it, carrying sand and other rocks downstream. Strong currents create hydraulic action that causes the softer rock to erode and dissolve.
Gemstones and gem-bearing rocks carried by rivers are called alluvial deposits and can be transported for many miles downriver. Keep an eye out for gravel bars in shallow parts of the river, especially where it bends.
Riverbanks, basins, and old riverbeds are ideal places to go rockhounding as the rocks are usually clearly visible, washed clean by the water.
Learn about local rivers and their geology in your area by consulting rockhounding guides, websites, and natural history museums. Keep an eye out for recently eroded cliffs and banks along rivers for fresh pickings.
TIP: You already know what rocks can be found in rivers so it is time to try to find some of these rocks. Do you know what tools you need to take with you? Check out this complete guide with all tools you need for rockhounding: