Michigan Rocks & Minerals: A Field Guide to the Great Lake State

By Dan R. Lynch and Bob Lynch

Get this must-have guide for Michigan, featuring full-color photographs and information to help you identify rocks and minerals.

Identify and collect rocks and minerals with the perfect guide to the Great Lake State! With this famous field guide by Dan R. Lynch and Bob Lynch, field identification is simple and informative. The book features comprehensive entries for 96 rocks and minerals, from common rocks to rare finds. That means you’re more likely to identify what you’ve found. The authors know rocks and took their own full-color photographs to depict the detail needed for identification—no more guessing from line drawings. The field guide’s easy-to-use format helps you to quickly find what you need to know and where to look.

Inside you’ll find:

• 96 specimens: Only Michigan rocks and minerals
• Quick Identification Guide: Identify rocks and minerals by color and common characteristics
• Range/occurrence maps: See where each specimen is commonly found
• Professional photos: Crisp, stunning images

Michigan Rocks & Minerals includes beautiful photography, relevant information, and the authors’ expert insights. With this book in hand, identifying and collecting is fun and informative!

• Language: English
• Publisher: Adventure Publications
• Release date: Jul 19, 2010
• ISBN: 9781591936664

Book preview

Hardness and Streak

There are two important techniques everyone wishing to identify minerals should know: hardness and streak tests. All minerals will yield results in both tests, as will certain rocks, which makes these tests indispensable to collectors.

The measure of how resistant a mineral is to abrasion is called hardness. The most common hardness scale, called the Mohs Hardness Scale, ranges from 1 to 10, with 10 being the hardest. An example of a mineral with a hardness of 1 is talc; it is a chalky mineral that can easily be scratched by your fingernail. An example of a mineral with a hardness of 10 is diamond, which is the hardest naturally occurring substance on earth and will scratch every other mineral. Most minerals, including many of Michigan’s, fall somewhere in the range of 2 to 7 on the Mohs Hardness Scale, so learning how to perform a hardness test (also known as a scratch test) is crucial. Common tools used in a hardness test include your fingernail, a copper coin, a piece of glass and a steel pocket knife. There are also hardness kits you can purchase that have a tool of each hardness.

To perform a scratch test, you simply scratch a mineral with a tool of a known hardness—for example, we know a steel knife is a hardness of 5.5. If the mineral is not scratched, you will then move to a tool of greater hardness until the mineral is scratched. If a tool that is 6.5 in hardness scratches your specimen, but a 5.5 did not, you can conclude that your mineral is a 6 in hardness. Two tips to consider: as you will be putting a scratch on the specimen, perform the test on the backside of the piece (or, better yet, on a lower-quality specimen of the same mineral), and start with tools softer in hardness and work your way up. On page 10, you’ll find a chart that shows which tools will scratch a mineral of a particular hardness.

The second test every amateur geologist and rock collector should know is streak. When a mineral is crushed or powdered, it will have a distinct color—this color is the same as the streak color. When a mineral is rubbed along a streak plate, it will leave behind a powdery stripe of color, called the streak. This is an important test to perform because sometimes the streak color will differ greatly from the mineral itself. Hematite, for example, is a dark, metallic and gray mineral, yet its streak is a rusty red color. Streak plates are sold in some rock and mineral shops, but if you cannot find one, a simple unglazed piece of porcelain from a hardware store will work. There are only two things you need to remember about streak tests: If the mineral is harder than the streak plate, it will not produce a streak and will instead scratch the plate itself. Secondly, don’t bother testing rocks for streak, since they are made up of many different minerals and won’t produce a consistent color.

The Mohs Hardness Scale

The Mohs Hardness Scale is the primary measure of mineral hardness. This scale ranges from 1 to 10, from softest to hardest. Ten minerals commonly associated with the scale are listed here. Some common tools used to determine a mineral’s hardness are listed here as well. If a mineral is scratched by a tool, you know it is softer than that tool’s hardness.

For example, if a mineral is scratched by a copper coin but not your fingernail, you can conclude that its hardness is 3, equal to that of calcite. If a mineral is harder than 6.5, or the hardness of a streak plate, it will have no streak and will instead scratch the streak plate itself unless weathered or altered by other, softer minerals.

Quick Identification Guide

Use this quick identification guide to help you determine which rock or mineral you may have found. We’ve listed the primary color groups and some basic characteristics of the rocks and minerals of Michigan, as well as the page number where you can read more about your possible find. The most common traits for each rock or mineral are listed here, but be aware that your specimen may differ greatly.

Agate

HARDNESS: 7 STREAK: White

Occurrence

ENVIRONMENT: Lakeshore, riverbeds, mine dumps, road cuts

WHAT TO LOOK FOR: Red or brown masses of material that contain concentric (bull’s-eye) banding and look and feel waxy

SIZE: Agates are rarely larger than your fist

COLOR: Brown to red, white, yellow, gray to blue; multi-colored banding

OCCURRENCE: Uncommon

NOTES: Agates are perhaps Lake Superior’s best known and most widely collected gem. While Minnesota’s North Shore is famous for agates, Michigan’s Lake Superior shorelines are also great hunting grounds for the popular stones. Agates are a banded variety of chalcedony, which is microcrystalline quartz (quartz crystals too small to see), that form within vesicles (gas bubbles) in basalt. These bands are like the layers of an onion—one on top of another, growing inward. This is known as concentric banding, or banding that resembles a bull’s-eye. Nevertheless, agates are very under-researched and while there are many theories, no one knows for certain how the bands form.

As a rule, if a specimen of chalcedony does not have concentric bands, it is not an agate. Agates exhibit all the traits of quartz-based minerals, such as a waxy surface feel and appearance, considerable hardness, and conchoidal fracture (when struck, circular cracks appear). They also share traits with chalcedony, such as translucency and reddish-brown colors caused by iron inclusions, making them easy to identify.

WHERE TO LOOK: Lake Superior’s shorelines on the Keweenaw Peninsula are great spots to look, as are the beaches near Grand Marais.

Agate, Brockway Mountain

HARDNESS: 7 STREAK: White

Occurrence

ENVIRONMENT: Lakeshore, riverbeds, road cuts

WHAT TO LOOK FOR: Gray or brown balls that contain characteristic agate banding within

SIZE: Brockway Mountain agates are commonly golf ball-sized and smaller

COLOR: Gray, brown, or reddish-brown on the outside; orange, red, yellow, white to tan on the inside

OCCURRENCE: Uncommon

NOTES: The agates found on and near Brockway Mountain, at the tip of the Keweenaw Peninsula, are a unique brand of agate. They generally lack the chlorite coating found on many of the Keweenaw Peninsula’s agates and are not the water-worn, waxy specimens found on the lakeshore. Instead, they are found as rough, sometimes almost perfectly round, hard nodules (round mineral clusters), with a brown or gray outer surface color. The interiors of these nodules are distinctly colored in pastel shades of orange, yellow and white, sometimes with very tightly spaced banding. These similarly colored bands lack contrast and can sometimes appear as one solid field of color unless you look very closely.

Brockway Mountain agates can be found over a large area at the tip of the Keweenaw Peninsula. Most are inland and must be dug out of the ground, though there are beaches near Copper Harbor where they can also be found.

WHERE TO LOOK: Look in the area between Eagle River and Copper Harbor on the Keweenaw Peninsula. Look in outcroppings of exposed rock as well as the lakeshore.

Agate, Copper-Banded

HARDNESS: 7 STREAK: White