A rock or mineral specimen which is unfamiliar to the collector may be identified by using the information in this booklet in either of two ways: (1) the reader may turn through the pages and compare his specimen with the photographs of others named there and read their descriptions until he finds a match for his specimen; or (2), the better way, he may classify his specimen first by the use of the determinative key which follows and be directed thereby to the pages in the book for confirmation of the name by the photographs, description, and discussion of the substance. The writer recommends the second method and has prepared this booklet on the assumption that the determinative key will be used.

The simplest and probably the best means of separating specimens of different rocks and minerals is on the basis of hardness, which means resistance to scratching. Crushing strength is different from hardness; therefore, in testing for hardness, do not attempt to pulverize. Merely determine if the specimen can be scratched with the substance indicated.

Determination of the mark or “streak” of a mineral when rubbed on a hard white rock or unglazed porcelain is demonstrated in the photograph on page 56.

A. Specimens that can be scratched readily with the THUMB NAIL.

1. Become muddy when rubbed with a wet finger.

Page

Shale 20

Fire Clay 24

Flint Fire Clay 26

Diaspore Clay 27

2. Crumble easily into hard sand grains.

Sandstone 30

3. Chalky, white, porous.

Tripoli 37

Weathered Chert 36

“Cotton Rock” Dolomite 15

4. Clear and glassy, or glistening white; may split and show glassy, flat faces.

Gypsum 63

B. Specimens scratched readily with a POCKET KNIFE, or IRON NAIL but not with the thumb nail.

1. Loose sand grains scratched off.

Sandstone 30

2. Granular, but grains are tightly interlocked; also “bubbles” or effervesces in dilute muriatic (hydrochloric) acid.

Limestone 10

Dolomite 10

Marble 15

Formed in a cave.

Cave Onyx 16

Travertine 16

3. Chalky white, porous.

Weathered Chert 36

4. Black.

Coal 49

Black Shale 20

5. Pebbles or gravel cemented together.

Conglomerate 54

6. Powder becomes muddy when wetted.

Hard Shale 20

Flint Fire Clay 26

Diaspore Clay 27

7. Intense red; leaves a red mark or streak when rubbed on a hard white rock or on unglazed porcelain.

Hematite 56

Iron Band Diaspore 58

Paint Ore 58

8. Yellow, brown, or black and leaves a yellow-to-brown mark or streak when rubbed on a hard white rock or on unglazed porcelain.

Limonite 57

Ochre 57

9. Heavy, black, leaves a black or brownish black mark or streak when rubbed on a hard white rock or on unglazed porcelain.

Manganese Ore 58

10. Heavy, with bright metallic luster, and lead-colored on a freshly broken surface.

Galena 59

11. Looks like rosin, or may be ruby-colored or black, but has a high resinous luster on freshly broken surface.

Sphalerite 60

12. Glassy luster; water-white, milky, honey-colored, pink, gray; may occur in six-sided crystals, sometimes pyramid-shaped; always breaks with flat glistening faces; always reacts in the lump with cold dilute muriatic (hydrochloric) acid.

Calcite 16

13. Like calcite above but may have a pink, pearly luster and curved crystal faces; reacts with cold dilute acid when powdered but not readily in lump form.

Dolomite 18

14. Opaque white, glassy or bluish, very heavy, lustrous on freshly broken surface; does not react with acid.

Barite 61

15. Flaky, micaceous like “isinglass”.

Mica 44

C. Specimens TOO HARD to be scratched readily on a fresh surface with a pocket knife or iron nail; weathered specimens may be slightly scratched.

1. Very fine-grained throughout, compact; occurs in nodules, pebbles; breaks with a slick, curved, oyster-shell-like (conchoidal) fracture.

Chert, if white, gray or stained yellow or red 34

Flint, if black 34

Agate, if banded 37

Petrified Wood, if it shows the grain or bark of wood 37

2. Granular like sandstone but extremely hard and breaks through the grains as readily as around them.

Quartzite 41

Quartzitic Sandstone 30

3. Fine-grained, dark green to dark gray to greenish black; occurs in boulders north of Missouri River and in the granite and porphyry country or southeastern Missouri.

Basalt 46

4. Very fine-grained, compact, pink, red, brown, gray; usually “freckled” or sprinkled with grains about 1/16 inch in diameter.

Porphyry 45

Rhyolite 45

Rhyolite Porphyry 45

5. Coarse-grained (BB-shot size to considerably larger), glassy luster where freshly broken; pink, red, grey.

Granite 38

Gneiss, like granite but banded; occurs in boulders north of the Missouri River 38

6. Coarse-grained, dark green, dark gray, greenish-black.

Gabbro 54

Diabase 48

7. Brassy, metallic, heavy; leaves a black to greenish black mark or streak when rubbed on a hard white rock or on unglazed porcelain.

Pyrite 51

Marcasite 51

8. Glassy fragments breaking with rough fracture, or may occur in six-sided crystals; clear, water-white, milk-white, gray or pink; in sand grains; in granite.

Quartz 41

9. Intense red; leaves a red mark or streak when rubbed on a hard white rock or on unglazed porcelain.

Hematite 56

Iron Band Diaspore 58

Paint Ore 58

10. Yellow, brown or black, but leaves a yellow to brown mark or streak when rubbed on a hard white rock or on unglazed porcelain.

Limonite 57