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