Home Page Overview Site Map Index Appendix Illustration About Contact Update FAQ


Rocks, Minerals, and Gemstones

In the beginning of the universe, there were no minerals and rocks (aggregate of minerals). It is through the cycles of condensation, melting, dissolving (in liquid), and precipitation that the individual atoms come together to form small crystals and then minerals. The processes work because different substance condensate or precipitate at different temperature, but they do not cleanly segregate one type of mineral from the others. That's why it is so valuable to have mineral of relatively pure compound weighed a few carat (called gemstone, 1 carat = 0.2 gm). Table 09-02 presents a very brief summary of the evolution of minerals on Earth.

Period (MYA) Environment Process(es) # of Minerals Examples
13600-Present Since reionization Supernovae 0 Heavy Elements
13600-Present Cool Envelope of Stars Condensation Dozen Silicate Particles,
Carbon Grains
4540-4400 Formation of Earth Melting, Collisions 200 Olivine, Zircon
4400-2000 Black Earth Melting, Weathering 1500 Beryl, Tourmaline
2000-700 Red Earth Oxidation 2500 Rhodonite, Turquoise
700-400 White Earth Glaciation Cycles
2500 Kaolinite, Ice
400-Present Green Earth Bio-chemistry 4400 Aragonite, Calcite

Table 09-02 Evolution of Minerals

Rocks, Minerals, and Gemstones Gemstones Worldwide Rocks are aggregates of minerals - usually several, but sometimes only one or two. Minerals are either free, uncombined native elements (such as gold, silver, and copper), or elemental compounds (such as silicates - metallic elements combine with the Si-O tetrahedral radical). Gemstones are minerals suitable for use in

Figure 09-06m Rocks, Minerals, and Gemstones [view large image]

Figure 09-06n Gemstones Worldwide [view large image]

jewelry after cutting and polishing (Figure 09-06m). They are rare, and therefore valuable, because their formation requires a highly unusual set of geological
circumstances. Scientific study of gemstones can yield information about the inner condition of the planet million and billion years ago. Figure 09-06n shows the worldwide distribution of some gemstones. The size of each symbol signifies the economic importance of the gems from a particular region. Gems are often found in areas of tectonic or volcanic activity, but some deposits seem to be located where there is no evidence of magmatism (the formation of igneous rocks from magma).

Silicon and Oxygen Types of Rock Since the Earth's crust composed mainly of Oxygen (46.6%) and Silicon (27.7%) for a total of 75%, the predominant compositions in minerals and thus in rocks are compounds such as quartz (SiO2), feldspars (XAl1-2Si3-2O8, where X can be either the elements Na, K, or Ca), and Mica (...Si3O10...) (see Figure 09-06o). There are three types of rocks according to the formation process (see Figure 09-06p, and Table 09-03). They are further sub-divided into different grain sizes and colors (light, medium, dark, not shown in the figure).

Figure 09-06o Minerals in Rocks [view large image]

Figure 09-06p Types of Rock

Type Formation Characteristic Composition Examples
Igneous Solidified from molten magma either at the Earth's surface (extrusive) or underneath (intrusive). The crystals can be very large (via slow cooling), and mostly have random distribution Basalt, Granite
Metamorphic Created when existing rock is chemically or physically modified by intense heat or pressure, e.g., in collision of crustal plates Have either wavy foliation (layer) or more random arrangement Gneiss, Schist
Sedimentary Formed from erosion, transportation and subsequent deposition of pre-existing rocks or other kinds of sediments May occur in layers, grains may be poorly held together Shale, Sandstone

Table 09-03 The Three Types of Rock

The identities of these three types of rock do not last forever. They run in a cycle (Figure 09-06q) as described below:
Rocks Cycle 1a,b. Magma (molten rock) inside the Earth's crust rises through cracks and cools slowly underground forming igneous rocks composed of minerals with fairly large crystal sizes, these are known as intrusive igneous rocks. When the magma erupts onto the surface, as through a volcano, it is termed lava, the rapid rate of cooling makes the extrusive igneous rocks to form with medium to very small mineral crystals.
2. Once on the surface, the forces of erosion and weathering produce smaller particles (sands), which accumulate and compactify by pressure from upper layers to become sedimentary layers (rocks).
3. When sedimentary and igneous rocks are subjected to intense heat and pressure such as in the collision of the crustal plates, they turn into metamorphic rocks. Some of these are uplifted back to the surface by tectonic action.

Figure 09-06q Rock Cycle [view large image]

Further increases in temperature and pressure melt the rock deep under the crust into magma to complete the rock cycle.

Usually, minerals in rocks are small in size and scattered randomly within. On rare occasions such as described below, it is possible to form gem-grade minerals in nature:
Gem Formation 1. SolutionPrecipitation - Near surface water becomes weak acid solution (with CO2 dissolved in it) in which many minerals are soluble. Gems will form as the water evaporates (Figure 09-06r (1)). Hot water from hydrothermal deep under are sometimes highly acidic or alkaline, making an even better solvent for more types of minerals. The slower rates of cooling and/or evaporation allow for larger crystals to form. Many of

Figure 09-06r Gem Formation [view large image]

the world's highest quality specimens and metal ores have come form such environments (Figure 09-06r (2)). It may also appear as veins in the cracks (Figure 09-06r (3)).
2. MeltCrystallization - This process is associated with the formation of igneous rocks. Large crystals can form in the intrusive type. The extrusive type generally not be expected to hold large crystals. Only rarely do larger gem crystals show up in a matrix of finer grained rock of this type.
3. VaporCondensation - Usually solid does not condense readily from the vapor phase. However, it does happen under special condition (such as frost on car windshields). If gases are trapped in bubbles within the lava, gems can crystallize upon cooling. Other pockets, which do not produce crystals originally, may later be invaded by surface water with mineral solution ultimately forming geodes or other similar formations.

Minerals are classified into groups according to the chemical properties as shown in Table 09-04 below:

Class Composition # Location Examples
Silicate Metallic elements + Si-O > 500 95% of all rocks Quartz SiO2, Garnet Mn3Al2(SO4)3
Carbonate Metallic elements + (CO3)-2 200 Marine and evaporitic settings Calcite CaCO3, Dolomite CaMg(CO3)2
Sulfate Metallic elements + (SO4)-2   Evaporitic settings, hydrothermal veins Gypsum Ca(SO4)H2O, Barite Ba(SO4)
Halide Metallic elements + halogen 100 Evaporitic settings Halite NaCl, Fluorite CaF2
Oxide Metallic elements + oxygen > 250 Precipitates on Earth's surface Ice H2O, Hematite Fe2O3
Sulfide Metallic/Semi-metallic elements + sulfur > 300 Metal ores Pyrite FeS2, Chalcopyrite CuFeS2
Phosphate Metallic elements + (AO4)-3, where A can be P, As or V   Phosphate in teeth and bones Pyroxmangite Pb5(PO4)3Cl, Bayldonite
Element Chemical elements and alloys   Mines Gold Au, Sulfur S, Silicides Fe3Si
Organic Carbon + hydrogen   Fossil fuels Hydrocarbons CnH2n+2, CnH2n, CnHn

Table 09-04 Mineral Classification by Chemical Properties

A mineral can be identified by several physical properties, some of them being sufficient for full identification without equivocation. Table 09-05 lists some physical properties used to evaluate a gem in the trade:

Property Definition Range Examples
Structure Defined by length of the crystal axes and the angles between them Seven systems Diamond (cubic),
Quartz (trigonal)
Hardness Mineral of higher hardness can scratch the surface of those with lower hardness Mohs hardness in 10 scales Calcite (3), Quartz (7)
Luster Surface interaction with light Seven kinds Pyrite (metallic),
Quartz (vitreous)
Color Determined by impurity or internal structure From red to violet including colorless Ruby (red),
Quartz (colorless)
Streak True color in powdery form Red to violet including white Pyrite (dark green),
Quartz (white)
Transparency The amount of light passing through Transparent, translucent, opaque Pyrite (opaque),
Quartz (transparent)
Cleavage The way a mineral may split apart along various planes Perfect, good, imperfect, none Euclase (perfect),
Quartz (none)
Fracture The way a mineral may break contrary to natural cleavage planes Conchoidal, sharp edges, fibrous, irregular Euclase (conchoidal),
Quartz (conchoidal)
Specific Gravity Mass of the mineral relates to that of an equal volume of water 1-2 (light), 2-4 (normal),
>4 (heavy)
Amber (1),
Quartz (2.6)

Table 09-05 Gem Identification by Physical Properties

Go to Next Section
 or to Top of Page to Select
 or to Main Menu