Chapter 2

Metamorphic rocks have been changed from their original state into something else. Heat, pressure, chemical action change the crystalline structure, the texture, even form new minerals. All kinds of rock can be changed.

A Word On Fossils

Perhaps you'll find rocks containing fossils--or even fossils by themselves. They should form a separate part of your collection.

Fossils are the remains--or the outlines--of former plant or animal life buried in rock. The older the rock, the simpler the plant and animal life it contains. Thus fossils can give a clue to the age of the rock strata.

Fossils can teach history. They tell us about plants and animals that are now extinct--the dinosaur, for example. They can also tell of ancient climates. Coral found in rocks in Greenland suggests it must have once been warm. Remains of fir and spruce trees have been found in the tropics.

How are fossils formed? Teeth, bone and wood don't last long in their original state. However, buried materials decompose, leaving a film of carbon as a fossil. This results in a leaf tracery, or the outlines of some simple animal. On a gigantic scale, this process of forming carbon has resulted in our great coal deposits.

Sometimes the buried material is gradually replaced by silica or other substances, making petrified objects. Wood can be replaced--cell by cell--by agate or opal from silica-bearing water. The result is petrified wood, the finest examples of which can be found in our Petrified Forest National Park in Arizona. This can happen to shells, too.

How about molds and casts of footprints of ancient animals? A brontosaurus might have stomped along in soft, warm mud eons ago. The mud hardened and later another layer of soft earth covered the print, preserving it.

COLLECTING

If you want to collect rocks and minerals just for the sake of having them, you can buy specimens. Many can be purchased for 25 cents to $1 each, while a rare specimen can cost hundreds of dollars.

The true pleasure is in finding your own samples. Later, when you have a good-sized collection, you can fill gaps by buying specimens or swapping extras with other collectors. You'll be amazed at the number of amateur collectors. Perhaps no branch of science owes more to the work of amateurs than mineralogy. Our great collection of minerals in the U.S. National Museum in Washington, D.C., was gathered almost entirely by two amateurs who devoted many years and much money to their hobby.

Where To Look

Look for pebbles by the roadside, in beds of streams and riverbanks. Go out into the country for ledges on hillsides. Every road cut, cliff, bank, excavation, or quarry shows rocks and minerals. Railroad cuts, rock pits, dump piles around mines, building sites--they'll all yield specimens. Some of the best mineral specimens collected in New York City came from skyscraper and subway excavations. Help a New England farmer clear his field and you'll have more rocks than you know what to do with.

As for reference books, many states publish guides to mineral deposits. Mineralogical magazines list mineral localities.

Tips For The Field

Don't try to collect too much at once. Work early in the day or late in the afternoon. A hot sun on bare rock can make you sizzle--especially if you're loaded with equipment and samples.

Here's the equipment to take: newspapers for wrapping samples, notebook and pencil, geologist's pick, cold chisel, magnifying glass, compass, heavy gloves, a knife, and a knapsack. Later on, you may want a Geiger counter for spotting radioactive rocks.

Be selective. Hand-sized specimens are best. If your sample is too large, trim it to size, showing its most striking feature to best advantage. When you wrap the sample in newspaper, include a note telling when and where you found it. This information will be transcribed to a filing card when you add the specimens to your display, so make it as complete and accurate as you can.

When you get home, clean specimens with soapy, warm water, applied with a soft brush. Soluble minerals like halite can't be washed, but should be rinsed with alcohol. A coat of clear lacquer will protect some samples against dirt.

Arranging Your Collection

Put a spot of enamel on the specimen. Write on the spot--in India ink--a catalog number and have this number refer to a card in a file drawer. The card should list date, place found, identification of specimen, etc.

Group your samples: metallic minerals, semiprecious stones, nonmetallic minerals. Display them on a shelf, or buy or build a mineral cabinet with partitioned drawers. For smaller samples, use a Riker mount with a glass top.

[figure captions]

A common rock

Here's the equipment to take: newspapers for wrapping samples, notebook and pencil, geologist's pick, cold chisel, magnifying glass, compass, heavy gloves, a knife, and a knapsack.

What Do I Have?

How do you identify specimens?

Get books and magazines on rocks and minerals. Many have colored pictures that help.

But identification is best made by noting the physical characteristics of the rock or mineral. For minerals, there's a hardness scale in which a mineral of the higher number can scratch a mineral of the lower number but not be scratched by it. The scale is: 1) talc; 2)gypsum; 3) calcite; 4) fluorite; 5) apatite; 6) orthoclase; 7) quartz; 8) topaz; 9) corundum; 10) diamond. Remember it by this silly sentence: "The girls can flirt and other queer things can do."

When on a trip, remember that a fingernail has a hardness of 2.5; a penny, 3; a knife blade, 5.5; and a steel file, 6.5. Use these to scratch your sample and you can get an approximate idea of its hardness.

You can buy a set of hardness points. They're pointed pieces of minerals set in brass tubes, each marked with its hardness scale. The set costs about $30 (half that if you assemble your own).

Other tests for identifying minerals include specific gravity (weight of mineral compared to the weight of an equal volume of water), optical properties and crystal form, color and luster. Minerals differ in cleavage and fracture (how they come apart when cut). They leave distinctive streaks on unglazed porcelain. Some are magnetic, some have electrical properties, some glow under ultraviolet or black light, some are radioactive, some fuse under a low flame while others are unaffected. Many studies with the dissolved mineral can identify it beyond doubt.

But most of these are too complicated for the beginner. As you read, look at pictures and samples, and talk with other rockhounds or leaders of mineralogy clubs, you'll get better at identifying rocks. Museum experts and your state's geologist can help, too.

[figure captions]

Specific gravity balance

Blowpipe analysis

GEMS FOR THE LUCKY FEW

If you're lucky, you'll find gems or semiprecious stones. Gems are the finer, more crystalline forms of minerals which are ordinarily less beautiful and spectacular. The true gems are diamonds, emeralds, rubies and sapphires. All others are semiprecious and ornamental.

Diamonds are pure carbon, but did you know that rubies and sapphires are corundum minerals--rare forms of alumina. In slightly different form, they'd turn up on emery paper.

Other stones you might find are the quartz gems: rose quartz, amethyst, rock crystal, agate, jasper, bloodstone. Or opaque gems such as jade, moonstone, lapis-lazuli, obsidian, and turquoise.

You don't have to find them. You can buy gems in the rough or in blanks, then cut and polish them to make your own jewelry or decorations. This takes practice, plus a cutting and polishing outfit, wood vise, maybe a diamond wheel. (Or you can join a lapidary club that might already have the equipment).

First learn to make cabochons--stones with round or curved surfaces. Then try cutting facets (or faces) in transparent gems. Learn by reading, working with an expert, trial and error. Making jewelry is fun, and collecting gems is as interesting as collecting rocks and minerals; it brings the world into your home. From the West come agates, jaspers, petrified woods; from the East, colorful marbles, serpentines, granites. Alaska, Idaho, Connecticut or Austria will yield dark red garnets. Fine moonstones come from Ontario; quartz crystals from Hot Springs, Arkansas, can be compared with similar ones from the Swiss Alps or Brazil.

Rock collecting is a hobby you can tailor to your taste. But whether you concentrate on an area close to home or travel across whole continents, you'll find that the pleasure and knowledge you gain from your collection are matched by the fun and adventure of the search.

[figure captions]

Drop sticks to hold stones

Diamond cutting wheel

[BACK COVER]

A Brontosaurus

Shell Oil Company

Revised 8/88

End of Project Gutenberg's Let's collect rocks & shells, by Shell Oil Company