Types of Rocks Found In Iowa: A Guide To The 8 Most Common You'll Spot

Iowa presents an awesome opportunity for collectors and amateur geologists alike.

In this article, you will learn about the most likely specimens you’ll look for in this state.

Types of Rocks Found In Iowa: A Guide

Introduction

Let’s get started!

Geodes are abundant throughout the central and eastern parts of the state.

With virtually all the gravel beds across the state containing Lake Superior agates and your chances of finding geodes containing amethysts or chalcedony extremely high, this is a rock-hounding paradise.

Due to its ubiquitous nature and world-renowned quality, the quartz geode has been designated the state rock of Iowa.

This is where you’ll find the world-famous geode site in the vicinity of Keokuk, Iowa.

It is located near the tri-state intersection of Illinois, Iowa, and Missouri.

The geodes in this region formed in the limestones and dolomites of the Mississippian-age Warsaw Formation.

While most of the geodes found here are very small, being only a few centimeters across, they have outer layers of white to gray to blue-gray chalcedony with tiny quartz crystal interiors.

You’ll find these geodes in the stream beds and adjacent soil, containing an interesting array of crystals, including:

Ankerite – A crystallized mineral containing iron and related to dolomite.

Aragonite – An orthorhombic mineral similar to crystalline calcium carbonate, also including calcite.

Calcite – One of the more common minerals also known as calcium carbonate.

Dolomite – A crystalized mineral comprised of carbonate of calcium and magnesium.

Goethite – A crystalized, anhydrous oxide of iron.

Gypsum – A hydrated calcium sulfate mineral.

Kaolinite – The crystalline form of kaolin.

Marcasite – A sulfide of iron similar in form and also resembling pyrite.

Millerite – A nickel sulfide mineral also referred to as capillary pyrites.

Pyrite – A common mineral, iron disulfide, also referred to as “Fool’s Gold”.

Sphalerite – A crystalline form of zinc sulfide, amongst others.

So, let’s see what rocks you can track down in Iowa:

1. Geodes

Geodes are rounded rock structures inside which you’ll find a cavity filled with various mineral materials.

The minerals within the cavity are often tiny quartz crystals underlain by multiple bands of translucent gray and white agate.

Common crystals found inside Iowan geodes include white calcite, purple amethyst, and agate, which occur as numerous colorful bands.

Rare geodes have blue gem silica, pink rhodochrosite, opal, and many other rare materials. Geodes of various sizes from under one centimeter to several meters long can be found.

You can find geodes in just about every location, including river beds. They are so abundant that you would be hard-pressed to miss one on any excursion.

To enjoy them, the geodes must be broken open using various tools.

Geodes are collectibles that can fetch a very high price for particularly interesting specimens.

2. Agates

Agates are translucent with a variety of microcrystalline quartz present. The colorful, better-quality agates are used as semiprecious stones.

Agates are some of the earliest stones to have been used as gemstones and examples that are thousands of years old have been found.

They are used for cabochons, beads, small sculptures, and various decorative objects such as paperweights and bookends.

The jewelry industry produces agate rings, earrings, pendants, and other jewelry items, including necklaces, and even marbles.

3. Jasper

Jasper is more of a gemological term rather than a name used by geologists.

Jasper refers to selected examples of opaque microcrystalline quartz.

This is generally used to produce cabochons, various sized and shaped spheres, tumbled stones, or other lapidary projects.

Jasper can be cut very accurately and accepts a beautifully bright polish.

Once cut and polished, the stone exhibits a wonderful color and pattern, which is the reason for its popularity.

4. Chalcedony

When you find a geode, your expectations are high that it may contain some wonderful collection of colorful crystals.

Disappointingly, this is not always the case.

Chalcedony is found inside a large number of geodes and is presented as a geometric matrix of microcrystalline quartz.

Chalcedony crystals are very small and not visible to the naked eye.

The tiny chalcedony crystals attach to the inner wall of the geode and all point outwards from the original seed crystals.

The tiny hemispherical crystals grow into and over one another, with the resulting botryoidal structure resembling a pile of grapes.

While these are interesting geodes to collect, they are not as visually interesting as some of the other more exotic specimens you will find and their utility is limited to collections rather than for use as an ornament or desk feature.

5. Petrified Wood

Petrified wood is formed as a result of plant material being covered by sediment over millions of years.

It transforms into fossils as it is surrounded by material that protects it from decay.

Oxygen and various organisms that would otherwise disturb or destroy the plants are prevented from reaching them through the compacted sediment.

The groundwater, which carries inorganic material like opal, flows through the buried sediment in which the plant material lies.

There it replaces the original plant material with a variety of dissolved solids.

These solids include pyrite, silica, and calcite.

What’s left is a fossil that exhibits all the structures of the original material, such as the bark, wood, and cellular structures.

You can find petrified wood in most areas below the soil or river beds.

Petrified wood is popular in lapidary work.

It can be cut into various shapes for jewelry, or sawn into blocks to make ornaments or utility items such as bookends.

Larger pieces can be cut into thick slabs for use as tabletops.

Other uses are for the faces of clocks, tumbled stones, cabochons, as well as centerpieces for many other craft items.

6. Quartz Crystals

Quartz is one of the most abundant minerals in the area and is extremely resistant to weathering.

It is also highly resistant to chemical and physical weathering.

Generally found in quarries and mines, you can also find it in river beds where smaller pieces are usually deposited.

Quartz vibrates at a very precise frequency, which is the reason for its use in the manufacture of timepieces.

Its luster and color make for beautiful gemstones and they are most sort-after in the jewelry industry.

Commercially, quartz is used in the making of glass.

7. Chert

Chert is the mineral form of silicon dioxide.

It is a sedimentary rock composed of microcrystalline or cryptocrystalline quartz.

Chert is mostly found with grey or black coloring but it can be reddish in color from the presence of iron oxides.

These cherts are known as Jasper.

Chert breaks with a conchoidal fracture which produces very sharp edges.

This made chert a useful stone for making cutting tools and weapons.

Also known as flint, chert was used in flintlock firearms to strike a metal plate, producing a spark that ignited the gunpowder.

No longer used for making tools or weapons, chert is an interesting addition to one’s collection.

It has very little commercial value outside of examples that have been fashioned into ancient tools.

8. Pyrite

Pyrite is a dark yellow mineral with a bright metallic luster.

This very common mineral is formed at high and low temperatures and can usually be found in small quantities, in igneous, metamorphic, and sedimentary rocks.

As with chert, pyrite can create sparks when struck against metal and other hard objects, which makes it an ideal material for flintlock firearms.

Pyrite’s nickname is Fool’s Gold due to its gold coloring and metallic luster.

With a high specific gravity, it is often mistaken for gold by inexperienced miners or prospectors.

However, the two minerals are present in each other’s company and in some deposits, pyrite contains enough included gold to make mining worthwhile.

Pyrite is found both above and below ground, so you can obtain examples of it in most rock-hounding locations.

It is important to bear in mind that all surface and subsurface rocks belong to someone, usually the owner of the land on which they are found.

However, the rights to rocks and stones might be transferred or sold to other entities.

These entities could be businesses such as mines or non-profit organizations, educational institutions, or even state-owned entities.

It is therefore important that you obtain permission and, in some cases, permits to collect rocks or stones.

The penalties can be harsh for the illegal collection of rocks.

You can join a club or book visits to mines and quarries where you can collect legally without the chance of being fined or even arrested.

I hope that the information we have provided in this article helps you to enjoy the amazing collection of rocks that are found throughout Iowa.