Polishing the Petoskey

Type of Lesson: Hands-on activity, Discussion

Time Needed: 45 minutes

National Standards Addressed

Earth and Space Science, Grades K-4: Fossils provide evidence about the plants and animals that lived long ago and the nature of the environment at that time.

Earth and Space Science, Grades K-4: The surface of the earth changes. Some changes are due to slow processes, such as erosion and weathering, and some changes are due to rapid processes, such as landslides, volcanic eruptions, and earthquakes.

Earth and Space Science, Grades 5-8: Fossils provide important evidence of how life and environmental conditions have changed.

Earth and Space Science, Grades 9-12: Geologic time can be estimated by observing rock sequences and using fossils to correlate the sequences at various locations. Current methods include using the known decay rates of radioactive isotopes present in rocks to measure the time since the rock was formed.

Life Science, Grades 5-8: A population consists of all individuals of a species that occur together at a given place and time. All populations living together and the physical factors with which they interact compose an ecosystem.

Quick Summary of Lesson

Students will wear away part of a Petoskey stone, create small piles of rock dust, and make an attractive reminder of the many concepts associated with the study of rocks, fossils and minerals.

Materials for each student

1 unpolished Petoskey stone
polishing compound
coarse (60 grit) and fine grit (150 grit) sandpaper
small piece of carpeting or polishing cloth


Have each student work individually following the instructions for the three parts of this activity.

Part I
1. The most important step is the selection of the stone to be polished. Select a stone that has a shape that is pleasing and then wet it to get a "sneak preview" of what it will look like when it's polished.

2. Many people prefer to only polish one face leaving the rest natural. If this look is for you, select a side that is free of indentations or depressions as it is very time consuming to sand them out.

3. Use the coarsest sandpaper (60 grit) to sand smooth the surface(s) that you are working on. Stop and rinse off the stone from time to time to check your progress. The surface should be free of nicks, pits or depressions, and should have a consistent luster across its face. If after wiping it and watching it air dry, whitish, rough spots appear - more sanding is needed.

4. When you are satisfied with your progress, wash and dry your specimen thoroughly, and proceed with Part II.

Part II
5. Use the fine sandpaper (150 grit) to sand smooth and remove the scratches naturally left by the coarser sandpaper previously used.

6. Rinse and examine your specimen to determine when all the scratch marks have been removed. Use a hand lens or magnifying glass to get a closer look.

7. When you are satisfied with your progress, carefully wash and dry your specimen. It must be totally free of grit, as just one sandpaper grain can spoil the finished product. You should now be ready give the lasting polish to your stone in Part III.

Part III
8. Look your specimen over one more time for visible scratches, as the polishing compound will not remove them. Go back to the fine sandpaper if needed. Next, place the rug piece or polishing cloth on your table. Sprinkle about 1/2 teaspoon of polishing compound into the center of your carpet square. Add a couple drops of water at a time until there is a thin cream on the surface of the carpet.

9. Rub the specimen in a circular motion until the desired polish is obtained. Add water to the carpet as needed to keep the specimen moving freely across the surface of the carpet. When you've rinsed and dried your stone and it looks like it's been lacquered or waxed, then you're done!

Notes to the Teacher

Here is some background information you may want to know before having students do this activity. The Devonian Period began about 417 million years ago and ended about 63 million years later. During this period a mass of reef-built coral limestone was deposited across the state of Michigan. The Petoskey stone is the state stone of Michigan and is made up of the fossil coral stone which came from this period.

Petoskey stones are fossils now preserved in the lime muds that harden to limestone. This makes Petoskey stones sedimentary rocks. They are records of the life that was once very abundant in the warm salt seas that covered Michigan at various times. Other types of shelled forms are preserved in sandstones and shales.

Suggested polishing compounds are tin oxide or cerium oxide. For another time: The polishing compound still on the carpet may be used time and time again. Just add more to the carpet as needed. By storing the carpet in a ziploc bag, it won't dry out and will be ready to go the next time you need it.

Classroom Management
The lesson may be presented in many different ways: as an introduction to weathering, as a closing activity for a rock, mineral and fossil unit, as an on-going free time activity, as a follow-up to mineral hardness activities, as a follow-up to acid testing of minerals, or as a part of a study of sedimentary rocks.

This activity can take hours depending on the degree of shine students want on their stone. Students will create a lot of rock dust during the initial stage. You may want to do this part outside. You may also want students to place the heavy grit sandpaper on top of a cushioned surface such as a spelling book. This deadens the noise and makes the stone easier to handle at first.

Depending on what you want to emphasize, you may want students to collect the dust created from sanding in a small box. The students will be surprised at how much dust they collect and how fast it collects. Save the dust for a hydrochloric acid test of carbonate minerals. Or discuss the results of their efforts and compare this with the constant wearing away of rock by water, wind, sand, and grinding that humans bring about by walking or driving on rock surfaces. Finally, you could discuss the hardness of the various polishing materials and why they wear away the stone. [Petoskey stone hardness = 3, sandpaper hardness=7]

Suggested assessment for this activity is to have students write a description of this project. The description should include: selecting a stone, its preparation, the sanding phases, the polishing steps and a drawing of the fossil coral. Have students use a hand lens to watch the features develop throughout the sanding/polishing process. You may want them to draw what they see at each stage. You may also want to ask these questions as part of a discussion or assessment activity. How does the rock wear away? How do we identify fossils in rocks by seeing only a small part of them?

Extensions of this activity include vocabulary that could be used in spelling lessons (fossil, Petoskey stone, Devonian period). Students could read and report on the Devonian Period during social studies class. Students could also use polishing materials to polish other rocks. Students may want to experiment with different sandpapers - wet/dry automotive sandpaper works well, but is more expensive. Adding a medium grit stage may improve results (coarse 50 or 60 grit, medium 80 or 100 grit, fine 150 to 200 grit).

Need More Information? Try Using Windows to the Universe

Please use these links for further ideas or more information:
Geologic Time
Introduction to Geology
Let's Take a Rock Apart
Silica Tetrahedron

Last modified June 19, 2008 by Lisa Gardiner.
The source of this material is Windows to the Universe, at http://windows2universe.org/ from the National Earth Science Teachers Association (NESTA). The Website was developed in part with the support of UCAR and NCAR, where it resided from 2000 - 2010. © 2010 National Earth Science Teachers Association. Windows to the Universe® is a registered trademark of NESTA. All Rights Reserved. Site policies and disclaimer.