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Eileen finds out what gives fine gemstones their brilliance.
Segment Length: 7:00

Teacher's Guides Index


Show Number 1411

How do gemstones get their colors?

What different factors control how a gemstone is colored?

[Apple Image] Getting Started

How do the natural shapes of gemstone crystals control what the final cut gem will look like? Visit a local jewelry store or gem shop and see what you can find out from the experts on how gemstones are cut.

Where do gemstones come from? Do you own any jewelry with gems in it? What makes the gems sparkle? What shape are they? What color are they? What do you think affects the color of a gemstone?

[Apple Image] Overview

There's nothing as eye-catching as a piece of fine jewelry covered with beautifully colored gemstones. Natural gemstones, however, are simply mineral crystals whose chemistry and structure make them look special. Of the 2,000 minerals identified in the world, only about 16 yield important gemstones. To be a gemstone, a mineral has to be beautifully colored, hard, quite durable, and, most of all, rare!

Color is the most obvious and attractive feature of gemstones. The color of any material is due to the nature of light itself. Sunlight, often called white light, is actually a mixture of different colors of light. When light passes through a material, some of the light may be absorbed, while the rest passes through. The part that isn't absorbed reaches our eyes as white light minus the absorbed colors. A ruby appears red because it absorbs all the other colors of white light - blue, yellow, green, etc. - and reflects the red light to the viewer. A colorless stone absorbs none of the light, and so it allows the white light to emerge unchanged.

Some minerals are idiochromatic, or "self-colored." Their colors are part of the chemical and physical makeup of the minerals themselves. Other minerals are allochromatic, or have some color added due to contamination by other chemicals.

Most raw gems (including diamonds) have a rough shape and a dull color. Only after they have been cut and polished do they take on that special glow that people have come to expect. A person who cuts gems is called a lapidary, but actually that person does very little cutting. Instead, a lapidary uses a variety of grinding wheels and grits to shape and polish the colored gems or stones.

Gems are usually cut to highlight their internal color or natural crystal shape. The two main cutting techniques produce either cabochons or faceted gemstones. Cabochons are stones that have been cut, ground into the shape of a dome, and then polished on the outer surface. This technique is used primarily for opaque stones like opals that don't let the light shine through.

Faceting is generally used with gems that are transparent. By grinding regular, flat surfaces in a predetermined geometric pattern on the outside of the gem, a lapidary turns a rough diamond stone into a brilliantly-sparkling gem.

When light enters a faceted gemstone, it is bent to a different angle. This is called refraction. The facets on the outside of the gemstone are positioned so that the light enters the stone from the top, is bent, and eventually is reflected back to the viewer, displaying the brilliance within the gem. Nature provides the gemstones, but it's human ingenuity that turns them into dazzling jewels.

[Apple Image] Connections

  1. Gemstones have been collected and traded for thousands of years. Has the process of mining and polishing changed much over time?
  2. What industries depend on natural gemstones for high-tech applications? How have synthetic substitutes helped them expand and grow?

[Apple Image] Resources

Federman, D. (1990) Modern jeweler's consumer guide to colored gemstones. New York: Van Nostrand Reinhold.

Hall, C. (1994) Gemstones. New York: Dorling Kindersley.

Kunz, G. (1989) The curious lore of precious stones. New York: Bell Publishing.

Matlins, A. (1984) The complete guide to buying gems. New York:
Crown Publishers.

Mercer, I. (1987) Gemstones. New York: Gloucester Press.

Earth Magazine. Published bimonthly by Kalmbach Publishing Co. Lists regional mineral and gem shows: (800) 533-6644.

Lapidary Journal. Published monthly by Lapidary Journal, 60 Chestnut Ave,
Suite 201, Devon, PA 19333-1312.

3-2-1 Classroom Contact: Crystals - they're habit-forming. Available from GPN:
(800) 228-4630.

Student Activity:
Gems Picture

Crystal Clear

Try growing your own "synthetic gemstones" to see the shapes they form.

[Apple Image] Main Activity

One of the things that makes gemstones look so spectacular is the way the light bends and bounces inside them. This light show is mainly due to the internal arrangement of atoms within the crystal's structure. As the chemistry of the mineral changes, so does the crystal shape, or "habit." In this activity, you'll see for yourself how different chemical compounds produce different-shaped crystals when you cook up your own "gems."


1. Fill each plastic cup about half full of hot tap water. In the first cup, begin stirring in spoonfuls of table salt until you can dissolve no more (usually about 68 spoons). Use a piece of masking tape and the marking pen to label this first cup NaCl. With a clean spoon repeat the procedure with the second cup, only this time use the Epsom salt. Label this cup MgSO4. In the third cup, stir in the alum powder and label it KAl(SO4)2.

2. Tie a paper clip on the end of each string. Wet one string with water and rub some salt crystals on it. Take a piece of aluminum foil and poke a small hole in the middle of it. Thread the free end of the salted string through the hole in the foil, and then cover the cup marked NaCl with the foil. Pull up on the string so that the paper clip is just touching the bottom of the cup and then secure the free end of the string to the top of the cup with a piece of tape.

3. Repeat step 2 first with the Epsom salt and then the alum powder. When all three solutions have been set up, place them in a safe location away from any direct heat or sunlight. In about one week, visible crystals should start to develop on the end of each string.

4. Using a magnifier, observe each crystal and draw a picture of its habit (shape). Compare the crystals you are growing to the three materials you started with and record your observations.

5. Allow the crystals to grow for another two to three weeks and use a millimeter ruler to keep track of their growth rate. Make sure that you note any changes in their appearance over time.


  1. What similarities and differences did you notice among the three crystals? How might their chemical formulas help to explain this?
  2. Why was it important to cover each of the glasses and keep them out of the sunlight?
  3. What might you do to reduce the amount of time needed to grow the crystals?

[Try This]
Gemstones are not found everywhere on earth. Research the connection between geography and gemstones and see if you can find out the geological reasons some areas are natural treasure chests.

[Try This]
Check with a local rock club or look in a lapidary journal to find out if there are places near your home to do mineral collecting. While they may not be gem quality, you can perhaps find your own gems in the rough as you build your collection.

[Try This]
Since diamond is the hardest substance known on earth, it occupies the top spot (#10) on the Mohs scale. On the other end of the scale is the mineral talc (#1), which is so soft it's used for baby powder. Research the Mohs scale and see where other gemstones rank. Also, see where materials like a penny, glass, and your fingernail fit in.

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