Rockin' gems: find out how your birthstone transforms from a diamond in the rough to a dazzling gem.

JANUARY GARNET SYMBOLIZES: LOYALTY

Several different kinds of garnet can be found in rock all over the world. The most common garnet is mined from within a gray rock called garnet schist. Sometimes weathering (wearing down by wind or rain) of the rock leaves chunks of garnet exposed on the rock's surface. After it is extracted from the rock, garnet is cut and polished to shine as a deep red gem.

FEBRUARY AMETHYST SYMBOLIZES: SINCERITY

To find this mineral (see Nuts & Bolts, p. 10), crack open a geode. These rocks have an inner cavity covered in crystals (solids in which the atoms are lined up in a repeating pattern). Material must have a crystal structure to be classified as a mineral. The crystal structure of amethyst makes it one variety of the mineral quartz. Its crystal pattern is the same as other types of quartz, like citrine, a yellow gem often mistaken for November's topaz. Amethyst's purple hue comes about when iron taints quartz.

MARCH AQUAMARINE SYMBOLIZES: BRAVERY AND WISDOM

Most minerals form underground from heat and pressure. Aquamarine typically forms when magma, or melted rock, cools in an underground pocket called a magma chamber. Rocks that form from the cooling of magma are called igneous rocks. The slower the magma cools, the larger the mineral crystals in the rock can become. "If it [cools and] grows for a long time, it will get huge," says Liz Nesbitt, curator of minerals at the Burke Museum in Seattle, Washington. Aquamarine is one variation of the common mineral beryl. May's emerald is another form of beryl.

APRIL DIAMOND SYMBOLIZES: INNOCENCE

Diamond is the hardest mineral on Earth. On the Mohs scale of hardness, which ranks minerals from softest (1) to hardest (10), diamond is a 10. It forms when carbon atoms crystallize under high temperature and pressure. What makes a diamond so tough? Its atoms are bonded strongly together in a tight crystal structure.

MAY EMERALD SYMBOLIZES: LASTING LOVE

Emerald is one type of the mineral beryl. Like most gems, its color comes from impurities that sneak into the mineral as it forms. Emerald's green hue comes from chromium atoms stuck in a beryl crystal. "Elements in the center of the periodic table tend to color minerals, making many ordinary minerals into gemstones (hard, colorful minerals)," says Mickey Gunter, a mineralogist at the University of Idaho.

JUNE PEARL SYMBOLIZES: HEALTH

Pearl is the only birthstone that's organic, or made by living things. When a particle of sand gets trapped inside an oyster or other shellfish, the animal coats it with a shiny substance called calcium carbonate. Over many years, layers of the material build up to form a pearl. And because they're made organically, pearls aren't minerals.

JULY RUBY SYMBOLIZES: CONTENTMENT

Ruby is a red form of corundum, a mineral of aluminum and oxygen atoms. Its densely packed atoms make corundum Earth's second-hardest mineral. Ruby forms during metamorphism, when heat and pressure underground rearrange the atoms that make up the minerals in a rock. "Imagine piling 10 miles of rock on your foot," says Gunter. "It's incredibly high pressure." The intensity can change a rock's looks and texture, and rearrange crystal structures to form new minerals in the rock.

AUGUST PERIDOT SYMBOLIZES: HAPPINESS

"Finding gemstones in rocks is a lot like looking for oil," says Gunter. "It's rare." And the rarer a gem is, the greater its value. Peridot is one variety of the mineral olivine, a major component of Earth's upper crust and one of the most abundant minerals on Earth. Olivine has rare varieties, including unique types of peridot. One--found only in Pakistan--has palm-size crystals. The most unusual peridot comes from meteorites (space rocks that hit Earth) called pallasites. Peridot is the only gem ever found in meteorites.

SEPTEMBER SAPPHIRE SYMBOLIZES: CLEAR THINKING

Like a ruby, a sapphire is a form of corundum. But sapphire is known for its blazing blue hue, caused by small amounts of iron in its crystal structure. It is found in metamorphic and igneous rock. Sapphire is also found in sedimentary rock. This rock type is formed when particles of other rocks and the remains of plants and animals become cemented together underground.

OCTOBER OPAL SYMBOLIZES: HOPE

Twirl an opal in the light and it shimmers with different colors. Like most gemstones, opal is translucent, or clear. One difference? Opal lacks crystals. It forms when silica (a compound made of silicon and oxygen) separates chemically from silica-rich water to form a solid. Opal contains 5 to 10 percent water. When light waves enter an opal, the water bends and scatters the light. This gives the gem its multicolors, explains Nesbitt.

NOVEMBER TOPAZ SYMBOLIZES: FAITHFULNESS

This golden-yellow gem is a silicate. It's made mostly of oxygen, silicon, and aluminum--Earth's three most-abundant elements. Like all minerals, topaz has a particular luster, or way in which light reflects off its surface. Geologists use luster to help identify a mineral. Topaz's luster is vitreous, or glassy. Other minerals may look metallic, earthy, waxy, or pearly.

DECEMBER TURQUOISE SYMBOLIZES: SUCCESS

Turquoise is an unusual gemstone. While most minerals are translucent, turquoise is opaque, or cloudy. And unlike other gemstones found in metamorphic rock, this one isn't formed under heat and pressure. Turquoise forms when water heated by magma flows into cracks in rock. The water changes the rock, creating a blue-green vein, or narrow slice of the mineral through rock.

Nuts & Bolts

All birthstones--except pearls and opals--are minerals. They are nonliving materials made by Earth's natural processes. Just as the elements on the periodic table combine to form different molecules, Earth's more than 4,000 minerals combine to form different types of rocks. "Minerals are the building blocks of Earth," says Nesbitt.

DID YOU KNOW?

* The size of a gem is measured in carats. A carat is a unit of measurement equal to 200 milligrams. The Star of Africa, one of the largest cut diamonds in the world, weighs 530.2 carats (106.04 grams). It's similar in size to a chicken's egg.

* Like pearls, amber is another organic gemstone created by a living thing--a tree. Some trees ooze a sticky, substance called resin. After the goo dries and hardens, it becomes the bright-yellow gem, number. Often, amber pieces contain fossilized bugs. The insects were trapped in the resin during its hardening process.

CRITICAL THINKING:

* No other natural substance on Earth is harder than a diamond. What are some of the possible uses for such a hard substance?

CROSS-CURRICULAR CONNECTIONS:

GEOGRAPHY: Each of the 50 U.S. states has a local flower, bird, and tree to represent the state. Some states even have state gents. Research to create a U.S. map that highlights these states. Be sure to include information about the chemical properties of each of these gems.

RESOURCES

* Gemstones, by Cally Hall, Dorling Kindersley, 2000.

* Take a virtual tour through the Smithsonian National Museum of Natural History's gem collection at: www.mnh.si.edu/explore/earth/index.html

(PAGE 8) ROCKIN' GEMS

DIRECTIONS: Defend or dispute the following statements. (Hint: Defend means to explain why a statement is correct. Dispute means to explain why a statement is incorrect.)

1. The Mohs scale measures the brightness of gemstones. For example: The ruby rates the highest number on the scale because it is considered the brightest gem.

2. Rocks that form from the cooling of magma are called sedimentary rocks.

3. Pearl is the only organic birthstone.

ANSWERS

1. Dispute: The Mohs scale ranks the hardness of minerals--from 1 for softest to 10 for hardest The diamond rates a 10 because it is the hardest mineral on Earth.

2. Dispute: Rocks that form from the cooling of magma are called igneous rocks Sedimentary rocks form when particles of other rocks and the remains of plants and animals become cemented together underground

3. Defend: Pearl is organic, or made by living things. When a particle of sand is trapped inside an oyster or other shellfish, the animal coats it with a shiny substance called calcium carbonate, Over many years, layers of the material build up to form a pearl. Pearls aren't minerals because they are made organically.