KEY TO THE MINERALS, BASED ON HARDNESS, COLOR, ETC.

OPAQUE COLORS
Color Hardness Streak Remarks Mineral
Red
scarlet 2.5 scarlet surface tarnishes black [prousite]
2.5 vermilion surface scarlet to dark red [cinnabar]
ochre 7 white non-crystalline [jasper]
6 ochre red color red to almost black [hematite]
rose 4 white effervesces in warm acid [rhodochrosite]
dark 4 orange [zincite]
2.5 purplish red surface tarnishes black [pyrargyrite]
brownish 3.5 brownish red [cuprite]
Orange 3.5 white to yellowish [pyromorphite]
1-1½ orange [realgar]
Blue 5.5-6 white in igneous rocks [sodalite]
azure 4 azure [azurite]
sky 7 & 4.5 white blade-like crystals [cyanite]
turquoise 6 blue non-crystalline [turquois]
2-4 white [chrysocolla]
Green
malachite 3.5 lighter green [malachite]
olive 6.5-7 white in igneous rocks [olivine]
3.5 white to yellow [pyromorphite]
2 white mica-like cleavage [chlorite]
1 white greasy feel, color light to dark olive green [talc]
yellowish 6.5 white [epidote]
2.5-4 white color yellow green to olive [serpentine]
Yellow
golden 2.5 shining non-crystalline [gold]
brassy 6 greenish-black usually crystalline [pyrite]
6 greenish-gray color pale brassy yellow, usually non-crystalline [marcasite]
5.5 greenish-black colors nitric acid green [millerite]
4 greenish-black color golden similar to gold [chalcopyrite]
3.5 dark brown purplish tarnish on surface [tetrahedrite]
bronze 5.5 pale brownish-black color with coppery cast [niccolite]
4 dark gray-black with speedy black tarnish [pyrrhotite]
3 gray-black brownish with bluish tarnish [bornite]
2.5 shining coppery red color [copper]
sulphur 3.5 white to yellowish compact masses [pyromorphite]
2 yellow [sulphur]
1-3 earthy masses [carnotite]
Brown
violet 1½ shining tarnishes black [cerargyrite]
yellowish 7.5 white 4-sided prisms [zircon]
6.5 gray [cassiterite]
5.5 ochre yellow compact to earthy masses [limonite]
5 brownish-yellow [goethite]
4.5 black [wolframite]
3.5 yellowish-brown [sphalerite]
3.5 white [siderite]
grayish 7.5 white often twinned [staurolite]
6.5 pale brown [rutile]
3.5 white to yellowish earthy masses [pyromorphite]
reddish 7 white dodecahedrons & trapezohedrons [garnet]
Black 6.5 gray [cassiterite]
6 reddish-brown [franklinite]
6 black magnetic [magnetite]
5.5 dark brown [chromite]
5.5 black yellow precipitate in sulphuric acid [wolframite]
5-6 black non-magnetic [ilmenite]
5-6 brownish-black compact masses [psilomelane]
5 brownish-yellow surface often brownish [goethite]
3.5 dark brown tetrahedrons [tetrahedrite]
2.5 silvery fresh surfaces silver color [silver]
2.5 scarlet fresh surfaces bright red [prousite]
2.5 purplish red fresh surfaces red [pyrargyrite]
2 black earthy masses [pyrolusite]
1 steel gray greasy feel [graphite]
Metallic Gray 2.5 black tarnishes black, bluish, or green [chalcocite]
2.5 lead gray sectile [argentite]
2.5 lead gray cubic cleavage [galena]
2 lead gray long prismatic crystals [stibnite]
1.5 bluish gray in scales [molybdenite]
steel 5.5 gray black rose color in nitric acid [smaltite]
4.5 steel gray very heavy [platinum]
4 reddish black often in striated prisms [manganite]
1 gray with greasy feel [graphite]
silvery 5.5 black [arsenopyrite]
2.5 silvery tarnishes black on exposure [silver]
reddish 5.5 gray black rose color in nitric acid [cobaltite]
pearly 1-1½ shining exposed surfaces violet brown [cerargyrite]
White, with impurities 4 white porcelainous masses, effervesces in acid [magnesite]
grayish or yellowish 2 white earthy masses, greasy feel [kaolinite]
1-3 white earthy masses [bauxite]
1 white greasy feel, fibrous or scaly [talc]
TRANSPARENT OR TRANSLUCENT COLORS
Color Hardness Remarks Mineral
Colorless or with faint tinges of color due to impurities
10 in octahedrons [diamond]
9 in hexagonal prisms [corundum]
8 in hexagonal prisms [topaz]
7 in three-sided prisms [tourmaline]
7 in hexagonal prisms [quartz]
7 non-crystalline [chalcedony]
7 or 4.5 cubes with beveled edges [boracite]
6 non-crystalline, pearly luster [opal]
5.5 rhombohedrons [willemite]
5.5 trapezohedrons [analcite]
5.5 tufts of needle-like crystals [natrolite]
5.5 sheaf-like bundles of crystals [stilbite]
5 hexagonal prisms with basal cleavage [apatite]
5 effervesces in acid [smithsonite]
5 becomes jelly-like in acid [calamine]
4.5 monoclinic prisms [colemanite]
4 in cubes [fluorite]
3.5 effervesces in acid, but one cleavage [aragonite]
3.5 effervesces in acid, heavy [cerrusite]
3 effervesces in acid, rhomboidal cleavage [calcite]
3 no effervescence, but soluble in nitric acid [anglesite]
2.5 in cubes tastes of salt [halite]
2 soluble in water, sweetish taste [borax]
2 1 perfect cleavage, and two imperfect cleaves at 66 with each other [gypsum]
White or with faint tinges of color due to impurities, such as pink, bluish, etc.
7 hexagonal prisms [quartz]
7 non-crystalline [chalcedony]
7 or 4.5 cubes with beveled edges [boracite]
6 non-crystalline, pearly luster [opal]
6 cleavage in 3 directions, good in 2 and imperfect in the other [feldspar]
5.5 short eight-sided prisms [pyroxene]
5.5 long six-sided prisms [amphibole]
5.5 trapezohedrons [analcite]
5.5 tufts of needle-like crystals [natrolite]
5.5 sheaf-like bundles of crystals [stilbite]
5.5 rhombohedrons [willemite]
5 effervesces in acid [smithsonite]
5 becomes jelly-like in acid [calamine]
4.5 & 7 cubes with beveled edges [boracite]
4.5 monoclinic prisms [colemanite]
4 effervesces in acid, porcelainous [magnesite]
3.5-4 effervesces in acid, heavy, red color in flame [strontianite]
3.5 effervesces in acid, heavy, green color in flame [witherite]
3.5 effervesces in warm acid, rhomboidal cleavage [dolomite]
3.5 effervesces in acid, cleavage in one direction only [aragonite]
3.5 effervesces in acid, heavy, does not color flame [cerrusite]
3-3.5 no effervescence, cleavage in three directions at right angles [anhydrite]
3 effervesces in acid, rhomboidal cleavage [calcite]
3 tabular crystals, heavy, green color in flame [barite]
2-3 cleaves in thin elastic sheets [mica]
2.5 cleaves in cubes [cryolite]
2.5 cubes, soluble in water, salty taste [halite]
2 1 perfect cleavage, and 2 less perfect ones [gypsum]
2 cleaves in thin non-elastic sheets [chlorite]
2 soluble in water, tastes sweet [borax]
1 greasy feel [talc]
Green 9 hexagonal prisms [oriental emerald]
8 octahedrons [spinel]
7.5 hexagonal prisms [beryl]
7 three-sided prisms [tourmaline]
7 dodecahedrons or trapezohedrons [garnet]
7 non-crystalline [prase] or [plasma]
6.5-7 non-crystalline, olive color [olivine]
6.5 yellow green color, rather opaque [epidote]
6 non-crystalline, pearly luster [opal]
5.5 short eight-sided prisms [pyroxene]
5.5 long six-sided prisms [amphibole]
5 hexagonal prisms [apatite]
4 cubes [fluorite]
3.5 effervesces in acid [cerrusite]
2.5-4 somewhat greasy feel, massive or fibrous [serpentine]
2 in mica-like scales, non-elastic [chlorite]
1 greasy feel, fibrous or scaly [talc]
Red 9 hexagonal prisms [ruby]
8 octahedrons [spinel]
7 three-sided prisms [tourmaline]
7 dodecahedrons or trapezohedrons [garnet]
7 hexagonal [rose quartz]
7 non-crystalline [jasper] or [carnelian]
6 pearly luster [fire opal]
4 cubes, rose tints [fluorite]
2-3 pink mica-like scales [lepidolite]
Blue 9 hexagonal prisms [sapphire]
7 & 4.5 blade-like crystals [cyanite]
6 non-crystalline masses [turquois]
5.5-6 in igneous rocks [sodalite]
4 azure color [azurite]
3.5 effervesces in acid, heavy [cerrusite]
2-4 earthy masses, turquoise color [chrysocolla]
Violet 7 hexagonal prisms [amethyst]
4 cubes [fluorite]
Yellow 9 hexagonal prisms [oriental topaz]
8 octahedrons [spinel]
8 hexagonal prisms [topaz]
4 cubes [fluorite]
Brown 9 hexagonal prisms [corundum]
8 octahedrons [spinel]
7.5 four-sided prisms [zircon]
7 hexagonal prisms [smoky quartz]
7 three-sided prisms [tourmaline]
7 non-crystalline [flint]
6 non-crystalline [opal]
5.5 short eight-sided prisms [pyroxene]
5.5 long six-sided prisms [amphibole]
2-3 cleaves into thin sheets [mica]
Black 9 hexagonal prisms [corundum]
8 octahedrons [spinel]
7 three-sided prisms [tourmaline]
5.5 short eight-sided prisms [pyroxene]
5.5 long six-sided prisms [amphibole]
2-3 cleaves in thin sheets [mica]

The Gold Group

Gold was undoubtedly the first metal to be used by primitive man; for, occurring as it did in the stream beds, its bright color quickly attracted the eye, and it was so soft, that it was easily worked into various shapes, which, because they did not tarnish, became permanent ornaments. The metal is associated with the very earliest civilizations, being found in such ancient tombs as those at Kertsch in Crimea and in northern Africa and Asia Minor. It was used in the cloisonné work of Egypt 3000 years B.C. In America the Indians, especially to the south, were using it long before the continent was discovered.

Of all the metals gold is the most malleable, and its ductility is remarkable, for a piece of a grain’s weight (less than the size of a pin head) can be drawn out into a wire 500 feet long; and it can be beaten into a thin leaf as thin as ¹/₂₅₀₀₀₀ of an inch in thickness, and thus a bit, weighing only a grain, can thus be spread over 56 square inches.

It forms very few compounds, but has a considerable tendency to make alloys (i.e., mixtures with other metals without the resulting compound losing its metallic character). In Nature gold is never entirely pure, but is an alloy, usually with silver, there being from a fraction of 1% up to 30% of the silver with the gold, the more silver in the alloy, the paler the color of the gold. Australian gold is the purest, having but about .3% of silver in it, while Californian gold has around 10% and Hungarian gold runs as high as 30% of silver. Another alloy fairly abundant in Nature is that with tellurium, such as calaverite (AuTe₂) which is a pale brassy yellow, similar to pyrite, but with the hardness of but 2.5. Another combination includes gold, silver and tellurium, sylvanite, (AuAgTe₄) a silvery white mineral with a hardness of but 2. Such combinations are known as tellurides and the calaverite is mined as a source of gold at Cripple Creek, Colo., while the sylvanite is one of the important ores of gold in South Africa. Occasionally gold is also found alloyed with platinum, copper, iron, etc. Jewelers make several alloys, “red gold” being 3 parts gold and 1 of copper, “green gold” being the same proportions of gold and silver, and “blue gold” being the combination of gold and iron. Our gold coins are alloys, nine parts gold and one of copper, to give them greater durability. Most of the gold recovered from nature is found native, i.e., the pure metal, or with some alloy.

[Gold]
Au
[Pl. 5]

Usually non-crystalline, but occasionally showing cube or octahedral faces of the isometric system; hardness 2.5; specific gravity 19.3; color golden yellow; luster metallic; opaque.

Gold is mostly found as the metal and is readily recognized by its color, considerable weight, hardness, malleability, and the fact that it does not tarnish. It usually occurs in quartz veins in fine to thick threads, scales or grains, and occasionally in larger masses termed “nuggets.” It is insoluble in most liquids so that when weathered from its original sites, it was often washed down into stream beds, to be found later in the sands or gravels, or even in the sea beaches. When thus found it is termed “placer gold,” and its recovery is placer mining. Most of the original discoveries of gold have been in these placer deposits; and from them it has been traced back to the ledges from which it originally weathered. In the placer deposits the size of the particles varies from fine “dust” up to large nuggets, the largest found in California weighing 161 pounds; but the largest one found in the world was the “Welcome Nugget,” found in Australia, and weighing 248 pounds. When gold was discovered in California in 1848, this became the chief source for the world, but later this distinction went to Australia, and now belongs to South Africa, which today yields over half the annual supply.