Lesson | Causes of Color

A page from the "Causes of Color" exhibit...

Lesson 2 (Gemstones)

Lesson

Ideally, this lesson on gemstones should coincide with or supplement your lessons on igneous rocks, or a lesson on rocks and minerals and their differences. Immerse your students in the world of color, and have them organize and communicate what they learn using SpicyNodes concept maps. What’s SpicyNodes? It’s a concept mapping and a presentation tool. It’s available as a free online tool, or your school can purchase an Enterprise subscription for the extra convenience of keeping track of your students’ work.

Over the course of this one-week lesson, you will guide your students through ideas about the causes of color and the color of gemstones. Let them research using this exhibit, perform hands-on activities, and create nodemaps that are intended for other students to use, and record some of their results to their notebooks or your usual classroom mechanism.

The following is a suggested lesson overview and notes. Please freely expand or shorten the lesson to meet the needs of your classroom.

Lesson overview

Day 1

Start the lesson with pictures of: rainbows, light bulbs, gemstones, a painting, and a laser light show. If you have a projector, you can use this WebExhibit; if you don’t, have students find pictures in magazines the day before you start. Ask questions: “What makes the colors?” and “Why do we see colors?” You can make a list of “What students know” Now, have students explore Causes of Color.

Start the lesson with an example of a simple nodemap on the projector or white board. Ask questions: “Why is a chart helpful?” and “What can you do with a chart like this nodemap?” Working in groups of two or three, have students create the beginning of their nodemap, “The Causes of Color.” Confirm that each student can do this activity.

  • Homework: Read about gold.
  • Before the lesson, we encourage you to make your own simple node map to get a feel for what your students are going to do.
  • To set up the nodemaps, go to group set up if your school has an Enterprise membership; or sign up if your school does not.
  • Explain that the nodemaps are a presentation tool, and should be written to the point of view of other kids. Just like journalists and teachers try to use interesting examples, so too should students. Explain to your students that the titles of nodes should be intriguing, such as probing questions, “Why are butterflies colored?” or provocative statements, “Gross! Bug juice in your candy!” or silly statements and rhymes, “Glittering gold,” followed by more serious explanations of the results of their hands-on experiments.

Day 2

Gold and Blue Diamonds

Review metallic bonding and wavelength measured in nanometers. Use the graph in the essay that explains the difference in the color of gold and silver because of the percentage of reflectance. Gold alloys are used because pure gold (24 carat) is so soft. Colloidal suspensions of gold nanoparticles yield different colors as shown on the last page of the essay on gold.

  • Use the color in blue diamonds page to explain that electrons drop from the conduction band to the valence band and emit energy in the visible region of the electromagnetic spectrum. There is a picture of the Hope Diamond.
  • Use the diamond band structure page to show how diamonds can be yellow because of nitrogen atoms, and blue because of boron atoms.
  • Homework: Read about colorless gemstones.

Day 3

Color in Gemstones

Introduce the nodemap they started, and have students add examples of “light lost” from this WebExhibit. Include a picture of the Hope Diamond. When they finish a node explanation for the Hope Diamond, they should add colorless gemstones to the nodemap.

Review Moh’s Scale of Mineral Hardness at a web site such as the one by Frederich Mohs. Beryl has a hardness of 8; corundum is twice as hard as beryl at a hardness of 9; and diamond is four times as hard as corundum at the greatest hardness of 10. Emerald and aquamarine are varieties of beryl; sapphire and ruby are varieties of corundum.

  • Colorless diamonds have an infinite array of carbon atoms in a rigid framework that is incapable of absorbing visible light. Corundum is made up of repeating units of aluminum oxide, and beryl is made up of repeating units of beryllium aluminum silicate. All the electrons are tied up in bonds and there is no absorption of visible light.
  • Review electron orbitals. If students have not studied chemistry, you may have to introduce them to s, p, d, ,f orbitals and electron arrangement. Rubies are red because of chromium atoms in the corundum, which become Cr3+ with partially filled orbitals. The electron energy from one orbital to another results in the emission of red light. Emeralds are green because there are chromium atoms in the beryl that become Cr3+ with partially filled orbitals. Energy differences between the energy levels are reduced, producing green light.
  • Homework: Read about blue sapphires.

Day 4

Blue Sapphires

Have students add nodes for emeralds and rubies to their nodemap, as well as sapphires. They can now add an explanation for the color of rubies and emeralds. Remind students that sapphires, like rubies, are basically corundum with impurities. The impurities in sapphires are titanium and iron, but the color cannot be explained by the crystal field theory; instead, it’s explained by a different process known as “charge transfer.”

  • When iron and titanium are both present in the aluminum oxide, electrons pass from one ion to the other and this transfer involves the wavelength of the energy that corresponds to yellow light. When the yellow light is subtracted from white light, the complementary blue color results.
  • There is a picture of ultramarine on the last page of the blue sapphire essay; this deep blue color is the result of anion-anion charge transfer,
  • Homework: Read gemstone colors from defects.

Day 5

Color Defects in Gemstones

Check student nodemaps for explanations of gemstone colors, including sapphires. Students have learned that colors in gemstones are caused by crystal field theory or charge transfer. Defects known as color centers within the crystal structure may alter its natural color. Defects are caused by heating or radiation. Topaz is an example of the effect of radiation.

  • Purple fluorite is pictured on the second page and amethyst on the third page of the essay. Amethyst has a color center in quartz, which is silicon dioxide - also known as sand. Amethyst crystals can be seen in the sand of North Shore beaches near Boston, Massachusetts.
  • Homework: Read DIY activity Charge Transfer in Iodine and Starch

Day 6

Activity Charge Transfer in Iodine and Starch

The directions are explicit; you can have the students work in groups of two or three and complete the activity as explained. You may need to supply some of the food in case students do not bring in samples.

  • You can assign this as a home activity, except that it is a good culminating activity for the week’s study of gemstones.
  • Another culminating activity is read ”What makes a gemstone”, summarize characteristics of gemstones, and learn the names of other gemstones.
  • You can give extra credit to students who can add nodes with the names of other classes of gemstones.

Other tips and activity ideas

  • Use photos! Pictures in a nodemap add interest and demonstrate creativity. Students can upload their own pictures of a light bulb in a jar, for example, or images from Google or Flickr.
  • Make word puzzles. Go to the page for creation of a word search puzzle using some of the vocabulary words.
  • Explore the vocabulary. During a Language Arts or writing time segment, have your students:
    • Group the words in the list as: light made, light lost, light moved, and vision;
    • Use some of the words in sentences;
    • Create a word scramble; or
    • Alphabetize the list.
  • Do some other cool color activities. Your students can try other DIY activities in this exhibit. See making bubbles, mixing lights, and triboluminescence with Wint-O-Green Life Savers.
  • For gifted and honors classes, suggest that they build a nodemap that involves five examples of each category: light made, light lost, and light moved, and vision. They may want to do other DIY activities from the WebExhibit.
  • Share your ideas and experiences. We’d love to have you send us a short note about your use of this exhibit, lesson, and SpicyNodes in your classroom. Let us know what worked, what didn’t, what you changed, and so forth. your comments to us, and we will publish your success story. We also encourage you to publish student nodemaps on your website or in the SpicyNodes Gallery.

Multidisciplinary connections

  • A Biology class can spend time on the structure and function of the parts of a human eye, the structure of chlorophyll and aspects of melanin, and color blindness. Biology classes may also find our exhibit Color Vision and Art interesting, in particular the background about ”What is Color?”
  • A Physical Science class can spend time on the electromagnetic spectrum and the relationship between frequency and wavelength, as well as the laws of reflections and refraction.
  • Environmental Science classes can study in detail the history of artificial lighting with an emphasis on compact fluorescent light bulbs.
  • Art classes can study the history of the color wheel and the difference between mixing lights and mixing paints. Art classes may also find our exhibit about Color Vision and Art interesting, in particular the Marilyn Monroe color changer.
  • A Chemistry class can study electrons and bonding and its relationship to color.
  • A Social Studies class can study the history of color representation.

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