Gems

 

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.

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." Materials

• 200 grams (7 oz) alum powder (available in a local pharmacy)
• 200 grams table salt
• 200 grams Epsom salt (available in a local pharmacy)
• 3 pieces of string, each about 15 cm (6") long
• 3 paper clips or washers
• 3 sheets of aluminum foil, approximately 30 cm (12") square
• 3 large plastic cups, at least 12 oz each
• hot tap water
• 3 plastic teaspoons
• roll of masking tape
• marking pen
• pocket magnifier

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. Questions

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?

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.

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