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1、Visit www.readinga- for thousands of books and materials.Gems:Treasures from the EarthA Reading AZ Level Y Leveled BookWord Count:2,097www.readinga-LEVELED BOOK YSVYWritten by Molly ChenGEMSTreasures from the EarthWritten by Molly Chenwww.readinga-GEMSTreasures from the Earth24Glossaryasterism(n.)a
2、star-shaped reflective pattern inside a gem(p.18)atoms (n.)the smallest units of a chemical element(p.6)crystals(n.)minerals formed in regular,tight patterns(p.5)cultured(adj.)made with the help of human beings(p.21)embedded(adj.)buried in;surrounded by(p.10)facets(n.)flat surfaces of a cut gemstone
3、 (p.11)fluctuate(v.)to change;to shift back and forth or up and down(p.13)gemologists(n.)scientists who study the chemical composition of gemstones(p.22)grain(n.)lines and patterns made by layers of minerals in a stone(p.9)lapidaries(n.)gem cutters(p.11)magma(n.)melted,liquid rock beneath the Earths
4、 surface(p.16)nacre(n.)the material oysters use to make their shells and to make pearls (p.9)opaque(adj.)not see-through(p.12)organic gems(n.)gems made from substances created by living things(p.5)polyps(n.)small sea invertebrates,such as coral,that have a tube-like body and a tentacled mouth(p.9)24
5、23ConclusionBeautiful gems can be found around the world.Many gems symbolize power and wealth.For centuries,people have killed and died for them.In some places,such as West Africa and Colombia,the gem trade is still filled with blood and violence.Mining gems with dynamite and strip mines is often da
6、ngerous,damaging both humans and the Earth.Scientists can grow gems in the lab that are identical to the finest natural gems but cost a thousand times less.In the future,created gems might help fill our desire for gemstones beauty while preserving Earth and its people.This strip mine clears an immen
7、se area of land.Gems:Treasures from the Earth Level YGems:Treasures from the EarthLevel Y Leveled Book Learning AZWritten by Molly ChenIllustrated by Cende HillAll rights reserved.www.readinga-Photo Credits:Front cover,back cover,title page,pages 4(bottom left),5(center,bottom),11(bottom),20(top),21
8、(main),22(all):Learning A-Z;pages 4(top right),10,17(bottom),19(top),20(bottom),23:Jupiterimages Corporation;pages 5(top),page 15:courtesy of USGS;page 8:iS Pintus:page 9:Colin Keates/Dorling Kindersley;page 11(top):REUTERS/Mark Baker;page 14:Gary Cook/Visuals Unlimited,Inc.;page 16:Steve Cole/Photo
9、disc/Getty Images;page 17(top):Smithsonian Institution/Corbis;page 18(left):iS 18(center):iS 18(right):iS 19(bottom):Siede Preis/Photodisc/Getty Images;page 21(inset):iS RobinsonCorrelationLEVEL YT4040Fountas&PinnellReading RecoveryDRA233Table of ContentsIntroduction.4How Are Gems Formed?.6Try This.
10、8How Do Gems Get into Jewelry?.10What Makes Gems Valuable?.13Hardness and the Mohs Scale .15What Are Some Types of Gems?.16Conclusion .23Glossary .24Gems:Treasures from the Earth Level Y22Gemologists are scientists who study the chemical composition of gemstones.Sometimes gemologists are experts in
11、a particular gemstone,such as diamonds or rubies.Gemologists who work in the laboratory can often identify which area of the world a particular gem came from by studying its chemical composition.They can also tell the difference between natural and synthetic gems by studying clues inside the gems th
12、emselves that give information about how the gem was formed.Natural pearls(inset)are rare and oddly shaped;cultured pearls are round.4IntroductionMany people consider gems to be Earths most beautiful creations.They are willing to spend thousands of dollars for even a small bit of that beauty.Sparkli
13、ng gems are worn on the fingers,necks,and wrists of people around the world.Families pass them down through generations.They can be found on crowns worn by royalty and on sacred religious objects.Many gems decorate the most beautiful and valuable jewelry.21Blue-green turquoise comes from the deserts
14、 of Iran,Tibet,and the southwestern United States.This stone often has pretty spots and streaks running through it.Much of the worlds turquoise is set in silver,as the Navajo Native Americans traditionally wore it.The Navajo believed turquoise to be pieces of the sky that had fallen to Earth.Natural
15、 pearls are strangely shaped and extremely rare.It takes an oyster many years to create a pearl from a tiny bit of sand.Almost all of the beautiful round pearls in jewelry stores are cultured,or made by people.Pearl farmers insert a round shell bead into an oysters shell.The oyster covers the bead w
16、ith nacre,creating a perfectly round pearl.Cultured pearls come in every color of the rainbow,from creamy white to pink to yellow to green and even black.Turquoise is often specked and striped with black.Gems:Treasures from the Earth Level Y5A gem is any beautiful stone that can be used in jewelry.M
17、ost gems have beautiful colors or sparkle,and they are hard enough to hold up to daily wear.There are three major types of gems.The first two,crystals and stones,are made of mineralsthe natural,nonliving substances that make up ordinary rocks.Crystals are very pure minerals that form in tight shapes
18、 and neat patterns.Stones are mixed minerals that have beautiful colors and patterns,but do not have strict shapes.The third group,organic gems,comes from substances made by living things.Garnet,a crystal(top);turquoise,a stone(center);natural pearl,an organic gem(bottom)Gems:Treasures from the Eart
19、h Level Y20Emeralds are known for their incredible green color.Gem-quality emeralds are rare and usually small,but people prize their color so much that emeralds are more valuable than diamonds.The finest emeralds are found in Colombia.Beryl,the same mineral that forms emeralds,also forms a blue-gre
20、en stone called aquamarine.Jade is one of the most prized stones.It occurs in lavender,white,and almost every shade of green,which is the most valuable.Jade is incredibly tough but easy to carve along its grain.Many civilizations,especially in Asia,used jade to make beautiful knives,swords,and axes.
21、Jade figurine(above);carving jade(left)6How Are Gems Formed?Some of the most famous and expensive gems,including diamonds,emeralds,and rubies,are crystals.Crystals are very pure forms of minerals.Every mineral is made of millions of particles called atoms,which are so small they are invisible to the
22、 naked eye.In ordinary rocks,many kinds of atoms are thrown loosely together without any kind of pattern or order.But in crystals,the atoms are arranged very precisely in neat,orderly patterns.Crystals have flat sides,called faces,which form shapes.Different kinds of crystals form in different shape
23、s,some of which are shown below.Some crystals form cubes,while others form long,six-sided columns.Examples of Crystal ShapesCUBICHEXAGONALMONOCLINICTETRAGONALTRIGONAL19The quartz family is the most common of all crystals.Quartz is found all over the world,in all colors of the rainbow.Most quartz is
24、so common that anyone can afford it.The largest quartz crystal ever found was about 6 meters(20 ft)long.It weighed more than 44,000 kilograms(48 tons)more than a loaded eighteen-wheeled truck.The most valuable quartz is a multicolored stone called opal.Opals shimmer with white,blue,and red-orange co
25、lors.They are most often found in Australia.Quartz crystals are common and can grow very large.Gems:Treasures from the Earth Level Y7Most crystals form deep within the earth under very special conditions.Some,such as diamonds,form when the mineral is squeezed under layers of rocks.The squeezing forc
26、es the atoms to arrange themselves until they are in the smallest shape possible.Others,such as sapphires,form when a mineral gets so hot inside the earth that it melts.As it slowly cools,the atoms fall into place to make a regular crystal pattern.And still other gems,such as opals,form when mineral
27、s dissolve in water.As the water evaporates very slowly,the mineral left behind forms a crystal.Diamonds are formed deep inside Earths mantle.Volcanic eruptions bring the diamonds to the surface.VOLCANOLAYERS OFEARTHSCRUSTPRESSUREFROMGRAVITYHEATHEATDIAMONDSMANTLEMAGMAGems:Treasures from the Earth Le
28、vel Y18Rubies and sapphires are made of the same mineral,corundum(aluminum oxide).Rubies,one of the rarest gems on Earth,must be truly blood-red or pink to be properly classified as rubies.The name ruby comes from the Latin word for red,ruber.The famous red color of rubies is caused by the element c
29、hromium.Any other form of the mineral corundum,no matter the color,is considered a sapphire.However,sapphires are most famous for their deep blue color.One feature that can occur in both rubies and sapphires is an asterism,or star.An asterism is a bright star-shaped pattern within the stone caused b
30、y titanium oxide that reflects light very brilliantly.Star sapphires and star rubies are highly valuable and some of the most sought-after gems in the world.Red ruby(left)and two sapphires,one blue and one yellow8 Make your own crystals!Rock salt,also known as halite,forms when salty seas evaporate.
31、You can watch a much smaller version of this same process using just water and ordinary salt.1 Mix a tablespoon of salt into a cup of warm water.Stir it until it dissolves.2 Keep adding salt,a little at a time,until no more salt will dissolve.3 Put a clean toothpick in the water.4 Place the cup some
32、where warm in the sun.As the water evaporates,crystals will form on the toothpick and the sides of the cup.5 Look at the crystals under a magnifying glass.What do they look like?Try This17One of the most famous diamonds,the Hope diamond,is not the largest,but it is an intense sky-blue color.Its long
33、 history of theft and ownership by royalty and the rich gave it a legend of being cursed.The largest diamond ever found is the Cullinan diamond,which was discovered in South Africa.It weighed over 3,100 carats and was as large as a pineapple.It was cut into nine gems,one of which,the Greater Star of
34、 Africa,weighs 530 carats.This photo of the Hope diamond shows its actual size.Do You Know?Graphite,or pencil lead,is exactly the same mineral as diamonds.In graphite,the atoms are arranged in loose layers.This makes graphite very softso soft that if you rub it on a piece of paper,the atoms break of
35、f,leaving a pencil mark.In diamonds,those same atoms have been squeezed together so tightly that nothing except another diamond can scratch them off.Gems:Treasures from the Earth Level Y9The minerals in stones do not form orderly patterns,and they may have other minerals mixed in.Stones often form i
36、n layers that make streaks and lines,called the grain.Grain gives stones beautiful patterns and surfaces.Organic gems,which include pearls,amber,and coral,come from living things.Pearls begin when a grain of sand gets trapped inside an oysters shell.The oyster covers the grain with layers of smooth
37、nacre,the material it uses to build its shell.Coral is made of skeletons left behind by millions of tiny sea creatures called coral polyps.Amber began millions of years ago when sticky sap oozed from trees and hardened.Amber often contains the fossils of insects and spiders that got caught in the sa
38、p.A spider in amberOysters create pearls.Gems:Treasures from the Earth Level Y16What Are Some Types of Gems?Diamonds form very deep in the earth as a result of huge amounts of pressure and high temperatures.Most diamonds form 140 to 190 kilometers(90 to 120 miles)beneath the Earths surface.Magma bri
39、ngs the diamonds close to the surface through volcanic pipes.In a few instances,diamonds have also been found in deposits left behind by melting glaciers.The diamonds that reach the Earths surface may be up to 3 billion years old.Diamonds are the hardest natural things in the world.Because diamonds
40、can cut anything,including metal and stone,flawed or unattractive diamonds are often put on saws and drill tips.Most diamonds are almost colorless,but very rare diamonds can be intense yellow,red,or blue.They are most often found in South Africa,Russia,and Australia.Diamonds are extremely popular fo
41、r engagement rings.10How Do Gems Get into Jewelry?Most mineral gems are found deep within the earth.Humans must dig mines to get them.Because gems are so small and rare,mining is often still done by hand.Miners must chip and cut at the rock,looking for individual stones embedded within it.It takes a
42、 lot of work to find gems,and work costs money.The rarer the gem and the harder it is to find,the more valuable it is.Do You Know?Opals form when mineral-rich water evaporates from cracks underground.This leaves a streak of mineral crystal in the rock.Streaks of crystal are called veins.The best opa
43、ls in the world are found in veins under the ground in the Australian desert town of Coober Pedy.In the Australian Aborigine language,Coober Pedy means“white man in a hole.”Opal mines are simple holes in the ground.15Hardness and the Mohs ScaleHardness indicates how pure and tightly structured the m
44、ineral is in a gemstone.It tells how well a gem will hold up to daily wear.A geologist named Friedrich Mohs developed a scale to test the hardness of gemstones.He simply scratched one gem with another.A gem can only scratch other gems that are softer than it is.The softest mineral,talc,which cannot
45、scratch anything else,is a 1 on the Mohs scale.Diamond,the hardest substance on Earth,can scratch anything,but can only be scratched by another diamond.Diamond is a 10 on the Mohs scale.Most gems must be 5 or above to be strong enough for use in jewelry.The Mohs scale measures the hardness of variou
46、s gems.For comparison,a fingernail has a hardness of 2.5,and a penknife measures 5.5.1 Talc 2 Gypsum 3 Calcite 4 Fluorite 5 Apatite 6 Orthoclase 7 Quartz 8 Topaz 9 Emerald 10 Diamond12345678109Gems:Treasures from the Earth Level Y11When a miner finds a gem,it looks very different from the one you se
47、e in a ring or necklace.The gem often has rough edges.Its surface looks dull.Its shape is bumpy.Gems often have cracks,dark marks,bubbles,and other flaws.Gem cutters,called lapidaries,cut gems into regular shapes that show off their best qualities and cover their flaws.Gem cutters once used diamond-
48、edged saws and polishers,but recently they began using lasers to cut gems.Most crystal gems are cut into flat surfaces called facets.Facets show off the gems color and pattern,and allow it to sparkle with reflected light.Lapidaries cut different gems using several different cutting styles.An uncut d
49、iamond looks uneven and flawed.A cut diamond reflects lots of sparkle.Gems:Treasures from the Earth Level Y14The next of the four Cs,color,is one of the major reasons why gems are considered beautiful.Unlike regular rocks,gems have bright,pure,intense colors.The stronger and purer the color,the more
50、 valuable the gem is.Often,the same mineral can form different-colored gems.Red rubies and blue sapphires both contain the same mineral.The different colors come from tiny bits of other chemicals mixed with the main mineral.It only takes one different atom in a thousand to change the color of a gem.