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Chapter 2 - Physical Properties of Gems

What is a crystal? A crystal is a solid substance with a homogeneous composition having a symmetrical geometric form with plane faces in symmetrical form.

What is Crystallography? Crystallography is the branch of science that studies the physical and chemical properties of crystals. Crystallographic studies typically focus on the internal arrangement of atoms within the crystalline structure of a gem, mineral, or chemical substance with an internal crystalline character. Most pure physical-chemical substances have at least one form of crystalline structure. Many substances have multiple crystalline forms related to the physical and environmental conditions in which they form.

Most gems are minerals that have unique arrangement of atoms in a crystal structure. The physical and chemical properties of the elements within the crystal structure give gems their unique properties!

Click on small images for a larger view.

Precious gemstones
Fig. 1-1. Classic gems.
Three factors play important roles in the physical properties of mineral:

1) the crystal structure,
2) character of chemical bonds within crystalline substances,
3) the ability of substances to split along cleavage planes.

How the arrangement of atoms affect physical properties is easily illustrated with two carbon minerals, graphite and diamond. In Figures 1-29 and 1-30, the lines between atoms represent chemical bonds. The structure of minerals and their bonding make a lot of the difference in the minerals’ properties, for example hardness.

These factors, particularly the hardness of a mineral and its tendency to split along cleavage planes, determine if and how a mineral specimen might be cut or faceted into a gemstone.
Crystal structure of diamond Crystal structure of Graphite
Figure 1-29. Crystal structure of the mineral diamond. Figure 1-30. Crystal structure of the mineral graphite.
Although both diamond and graphite consist of the element carbon, the two minerals have different crystal structure arrangements.

How Many Crystal Shapes Are There?

Well over 4,000 different minerals have been identified occurring naturally in the world. There are probably many more. Hundreds of thousand of inorganic compounds are known (and patented) and perhaps millions of organic compounds exist (having carbon and hydrogen and other elements combined. However, with all the chemical compounds that are known, there are only a relatively low number of naturally occurring, common or "important" mineral compounds that are gems or have "economic" significance.

Figures 1-31 to 1-36 illustrate a classification of natural crystal forms and shapes (grouped within crystal systems). Minerals have characteristic crystal shapes that can be used to help identify them.

Figure 31.
Cubic and
crystal system
Crystal forms:
include cube, octahedron, dodecahedron, and other more complex forms.

Cubic and Isometric crystal forms and examples of minerals
The Cubic or Isometric System include all crystal shapes that have symmetry axes in equal lengths in 3 directions (at 90º angles to each other). Common minerals that have a cubic/isometric crystal form include halite, fluorite, galena, pyrite, magnetite. Gem minerals diamond, garnets, spinel, and gold.
Figure 32.
crystal system.
"Rectanguloid" shapes, prisms, pyramids, and complex forms.
Tetragonal crystal forms
The Tetragonal System includes all crystal shapes that have three axes of symmetry all at right angles (90º) of each other. However, two sides of the crystal axes share equal length, whereas the length of the third axis is either shorter or longer than the other two. Some examples of minerals include apophylite, cassiterite, sheelite, and vesuvianite. Gems include zircon and rutile.

Figure 33.
crystal system
: six-sided prism, pyramid-shaped, rhombahedral, and combined forms. Both calcite and quartz produce a variety of crystal shapes within the hexagonal or trigonal forms.

Hexagonal and Orthorhombic crystal systems
The Hexagonal or Trigonal System includes crystal shape that are hexagonal. Three of the crystal axes are of equal length and lie in planes that are 120º from each other. The fourth axis is perpendicular (90º) to the three axes and is either shorter or longer to the other axes. Minerals with hexagonal form include calcite, dolomite, hematite, ice, quartz, and siderite. Gem minerals include beryl (including emerald), corundum (including ruby and sapphires), quartz varieties (crystal, citrine, amethyst), and tourmaline.
Figure 34. Orthorhombic
crystal system
: prisms, pyramids, and combined forms.
Orthorhombic crystal system
The Orthorhombic System includes crystal shapes that have three axes of equal length but all at right angles (90º) of each other. Minerals with orthorhombic forms include aragonite, barite, celestite, cerrussite, enstatite, olivine, stilbite and sulphur. Gem minerals include peridote (olivine) and topaz.
Figure 35.
crystal system
Monclinic and Triclinic crystal systems
The Monoclinic System includes crystal forms that have three unequal axes; two of the axes are at right angles (90º) but the third axis is inclined at an angle not at 90º. There is one two-fold axis of symmetry. Mineral examples include azurite, malachite, gypsum, epidote, amphiboles, jadeite, micas, and orthoclase.
Figure 36. Triclinic
crystal system
Triclinic crystal system
The Triclinic System includes crystal forms where the three axes are of unequal length, and one of the axes are perpendicular to each other. Mineral examples include kyanite, axinite, rhodonite, and albite.

How can physical and chemical properties of minerals be used for their identification?

All minerals have unique properties that aide in their identification. Some minerals have "unique" characteristics that have an appearance or characteristic that make them easy to identify. However, these identifying characteristics may not be easy to determine without extensive testing more extensive testing. Fortunately, the most common minerals are fairly easy to identify by general appearance or with simple tests for hardness, crystal form, color, magnetism, and "streak" (does it leave a colored line when scratched on a piece of tile?). Note that some tests can be destructive to mineral samples (such as measuring hardness, streak, malleability, elasticity, and testing with acid). In addition, tasting a mineral is not recommend - some are actually poisonous! Washing your hands after handling mineral samples is always recommended.

Observable Characteristics and Tests for Identifying Minerals

Easily Observable Characteristics Simple Tests Requiring Equipment
crystal form
luster (metallic, non-metallic)
diaphaneity (transparent, translucent or opaque)
double refraction
odor (smell)

acidic reaction
specific gravity
electrical resistivity fluorescence

Properties of minerals

The following physical properties of minerals can be used to identify a mineral through sensory observations or conducting simple tests. Equipment for such tests are typically available in science education departments or are available from commercial sources.

Easily observable physical characteristics (simple visual observations of the form and character of some minerals)
crystal form—many minerals have unique and sometimes obvious crystal structures, however, crystal structure alone may not be enough to identify a mineral. For most samples used in mineral tests, crystal form may not be apparent or easily measurable.
Fig. 37. Amazonite is a blue-green form of microcline feldspar. Samples of feldspars are fairly easy to find or purchase, and they typically have good crystal form (angles) for students to measure.
color—some minerals have very distinct colors, however, color is not a reliable indicator by itself. earthy luster
Fig. 38. Some minerals have obvious color associations. The combination of color with other mineral characteristics make the easy to identify: malachite (green), sulphur (yellow) and cinnabar (blood red). Problems arise with mineral samples are white or gray - there are dozens of minerals that have those neutral tones and make them difficult to easily identify without other tests.
cleavage—the tendency of a crystallized substance to split along definite crystalline planes, yielding smooth surfaces. Mica, feldspar, calcite, and selenite gypsum have good mineral cleavage. Flat, smooth, shiny and reflective surfaces on specimens may be either crystal surfaces and/or cleavage. muscovite mica has excellent cleavage
Fig. 39. Many minerals have cleavage planes that make them easy to identify, with micas (biotite is black, muscovite is silvery-white) being perhaps the most easy to recognize. Crushing irregularly shaped samples may demonstrate repeatable shapes associated with cleavage planes, such as with feldspar and calcite.
striations—some mineral crystals have fine, narrowly-spaced lines on crystal surfaces. (Examples of minerals that may display striations include hornblende, pyrite and selenite (a crystalline form of gypsum).
Pyrite crystal with striations
Fig. 40. Mineral crystals that grow in open cavities sometime display striations that are parallel to the crystal axes within the mineral's crystal structure. This sample shows a pyrite crystal with obvious striations. Note that striations may not occur on all all examples of a mineral. For example the cube-shaped pyrite specimen shown in Figure 41 does not display striations.
luster—the description of the quality and intensity (sheen or shine) of light reflected off of a mineral, particularly a reflective appearance of the exterior of crystal surfaces and cleavage planes.. There are many kinds of luster:
  • Metallic means having the appearance of polished metal. Native copper, gold, silver, and platinum have metallic luster on polished surfaces. Metalloid minerals including galena and pyrite have high metallic luster (Figure 1-41).

  • Earthy means having a dull or matte like appearance, like the texture of a terra cotta flower pot. Minerals like hematite and limonite that typically consist of very fine microscopic crystals have an "earthy" (dirt-like) texture. Samples in Figure 1-38 show an earthy luster

  • Adamantine means "having the hardness or luster of a diamond." Clear diamond is a highly "radiant" in bright light. Other minerals with high radiance include cubic zirconium, and "Herkimer diamond" (a unique variety of very clear quartz crystal). Most of the gems in Figure 1-1 display an adamantine luster.
  • Chatoyancy is the character of having a fibrous texture as seen in tiger’s eye. Tiger’s eye has fibers embedded in quartz and has a strong chatoyancy, but other minerals such as tourmaline and cat’s eye (chrysoberyl), or chrysotile also show this.
  • Schiller is luster property best seen in labradorite feldspar that varies in color as the mineral is moved and looks like the wings of some iridescent butterflies. Labradorite makes an attractive building material and semiprecious stone. Schiller is also seen in some gems such as moonstone.
  • Pearly luster as seen in variety of gypsum (called "satin spar") and ulexite (sometimes called the TV stone).
  • Greasy luster as in some chalcedony, a type of microcrystalline (also called cryptocrystalline) quartz.
  • Vitreous luster as seen in broken glass. On fresh, broken surfaces it has a conchoidal fracture pattern, like broken glass. Quartz crystals have a vitreous luster on broken surfaces.

  • Resinous luster as seen in amber (a fossilized tree resin; not a mineral)

    Luster is examined in more detail in Laboratory Exercise 1.
pyrite and galena
Fig. 1-41. Pyrite (left) and galena (right) have a metallic luster.
tiger eye quartz
Fig. 1-42. Tiger eye (a variety of quartz) displays chatoyancy luster.
Labradorite displays schiller luster
Fig. 1-43. Labradorite (a variety of feldspar displays a schiller luster.
Satin spar gypsum Chalcedony Obsidian Amber  
Fig. 1-44. Satin spar, a variety of gypsum displays a pearly luster. Fig. 1-45. Chalcedony, a variety of quartz has a greasy luster. Fig. 1-46. Obsidian, a natural glass, has a vitreous luster. Fig, 1-47. Amber has a resinous luster. It is actually fossil tree resin!
transparency—or more correctly, diaphaneity, is an evaluation of how light passes through a mineral, with general descriptions of being transparent (meaning clear enough for an object to be seen through a sample); translucent (a substance transmit light but it is dispersed or cloudy in appearance), or opaque (a substance will not transmit light). Few common minerals are transparent. Quartz and calcite can have high transparency (see Figures 1-48 and 1-50). Common milky quartz is typically translucent (light passes through but is diffuse, see Figure 1-49). quartz crystal milky quartz
  Fig. 1-48. Crystal quartz is transparent when clear. Fig. 1-49. Milky quartz is translucent.
double refraction—light passing through clear calcite ("iceland spar") will transmit a double image. Clear calcite can split a laser beam into two separate beams. Figure 1-50 shows a piece of iceland spar causing the X pattern of the underlying paper to be doubled on itself. Calcite "iceland spar" "fiber optic" properties—a notable example is ulexite, a soft borate mineral moves images from one side of a cut sample to the other side with a cut surface. Figure 1-51 shows the X pattern on the underlying piece of paper transmitted to the surface of the ulexite sample. Ulexite
Fig. 1-50. Clear calcite displays double refraction. Fig. 1-51. Ulexite (called "TV Rock") shows fiber-optic like properties.
Non-visual sensory characteristics of minerals
feel—The "feel" of a rock is not a reliable method of testing minerals, however certain minerals have textures like "soft, silky, satin, smooth, hard, heavy or light" - but these characteristics are poorly definable as a reliable means for identifying minerals.

odor—few minerals have an odor. Sulfur-bearing minerals may put off a rotten-egg like smell. Many rocks of sedimentary origin have the smell of petroleum.

—halite tastes like salt (because it is NaCl). (Note that tasting minerals and rocks is not recommended!)

Simple Tests For Identifying Minerals

Minerals have a variety of physical and chemicals properties that can be evaluated using simple tests. The following tests are simple determinations using common laboratory equipment and supplies. Note that some of these are destructive to samples being tested!
hardness—minerals have different durability properties. Depending on mineral chemistry and crystal structure, minerals have varying degrees of hardness. Simple tests of scratching mineral samples with items or material of known hardness can give a general range of "hardness" of a specimen.

Mohs Hardness Scale
is a list of hardness of common minerals. Note that testing the hardness of minerals may be destructive to samples! Mineral hardness and Mohs Hardness Scale are discussed and used in both Laboratory Exercises 1 and 2.
Mohs Hardness Scale
Fig. 30. Mohs Hardness Scale
magnetism—iron (naturally pure iron) and magnetite (Fe3O4) are common magnetic rocks, iron-rich meteorites are also magnetic. (This sample of Diablo Canyon meteorite is highly magnetic).

Many minerals rich in iron are partly magnetic and display measurable magnetic susceptibility that can be useful for geophysical exploration. Large bodies of rock containing iron-rich minerals can be remotely detected below the earth surface, and may be useful for detecting hidden faults, water-filled sedimentary basins, or potentially economically valuable mineral resource deposits. Magnetic susceptibility measurement are used in regional geophysical mapping.
Meteorite with a magnet attached
Fig. 1-52. Magnets stick strongly to magnetite (Fe3O4) but also show weak attraction to other metallic and metalloid minerals including hematite, goethite, chromite, franklinite, pyrrhotite, and siderite. Shown here, a magnet sticks strongly to a meteorite composed of the metallic iron-nickel mineral crystals (kamacite and taenite).
specific gravity—a measure of the "density" of a mineral. Specific gravity is the ratio of the density of a substance to the density of water. Tests for specific gravity require some laboratory equipment. Specific gravity tests are described and used in Laboratory Exercise 2. density illustrated
Fig. 1-52. Two equal size cubes with dots representing atoms. The box on the left has fewer "atoms" in the same amount of space as the second box. The second box would therefore be denser than the first box.
streak—soft minerals may leave a streak of color on a piece of tile. Hematite is red, pyrite is brown, magnetite is black, etc. Be aware that streak tests can be destructive to mineral samples. Streak is examined in Laboratory Exercise 1.
fluorescence—some minerals glow colors under a blacklight including some fluorite, calcite, and zinc minerals. Different minerals glow brightly (fluoresce) under different wavelength of ultraviolet light, sometimes in different colors under different wavelengths. The crystal structures of fluorescent minerals allow ultraviolet energy to be absorbed and the energy is released in a visible color wavelength (see Figure 1-53). Most rocks and minerals are not fluorescent.

phosphorescence—some minerals absorb light energy and release light when the light is turned off. Some varieties of calcite, zinc minerals, and minerals rich in phosphorus sometimes display phosphorescence. Phosphorescence is only observable in a very dark setting - very shortly after energy source (visible light, or better, ultraviolet light) is shut off. In most cases, the phosphorescent glow ends quickly. Some phosphate-rich calcites and zinc minerals can glow for quite a some time after being exposed to a light source, with brightness decaying slowly over time.
Zinc and calcite minerals under normal light Zinc minerals and calcite under short-wave ultraviolet light
normal light
short-wave ultraviolet light
Fig. 1-53. Fluorescent minerals from Franklin, NJ. Under short-wave UV light calcite glows red, and wilmenite and other zinc minerals glow green.
thermoluminescence—some minerals will glow in colors when heated, similar to a hot burner on a stove or an object held under a torch flame. Note that heating gems and minerals samples can (probably will) alter or destroy them.
radioactivity— Radioactive elements that occur in rocks and minerals include potassium, thorium, radium, and uranium. and may display measurable radioactivity. Most mineral samples do not have measurable levels of radioactivity. However, many older collections in science departments may have radioactive mineral samples, and these should be clearly identified and not handled. Radiation, like magnetism and gravity, are used in geophysical mapping and resource exploration. Granitic rocks tend to be slightly more radioactive than other rocks having trace concentrations of uranium or thorium. Fossil wood from the Colorado Plateau region can sometimes be radioactive. It is advisable not to collect radioactive material because of the potential health risks. If collected, they should be clearly marked and stored in appropriate containers. They may be illegal to own or transported. Radioactivity measured with a geiger counter
Figure 1-54. A geiger counter us used to measure materials for radioactivity. The sample shown here is a piece of gold ore from the Witwatersrand Gold Mine in South Africa. The gold is mixed in with uranium-bearing minerals and quartz. Many locations where gold occurs there may be other heavy elements, including uranium.
acidic reaction—Calcite fizzes when exposed to mild acid. Dolomite will fizz in hot acid.

Note that all minerals are chemicals that can react to chemical agents, altering or destroying them. Whereas gemstones are typically durable, the can be susceptible to chemicals added to cleaning fluids. Iron-bearing mineral will react to oxidizing compounds like bleach. Be sure to check on appropriate cleaning agents before cleaning gemstones or gem-bearing jewelry

calcite reacts with acid
Figure 1-55. Some minerals will react to exposure to acid. Calcite fizzes when exposed to hydrochloric acid or vinegar (acetic acid). Dolomite will fizz only in hot acid. Note that acid will not only destroy mineral samples but can also ruin clothes!
malleability—metals like gold, copper, iron, and silver is able to be hammered into objects.

elasticity—soft minerals may be bendable (like mica); most minerals fracture or shatter when put under stress or shock.

Figure 1-56. Whereas it is sometime fun to smash things, it is not really a useful means of testing minerals.
electrical resistivity—all native metals (gold, copper, silver) and many metalloid (metal-bearing) minerals will conduct electricity. Most metal ore minerals will conduct electricity. Common examples include iron ores: hematite, magnetite, pyrite, chalcophyrite, bornite, galena. Minerals with a metallic luster will conduct electricity. Conversely, non-metallic minerals will not conduct electricity.
electrical conductivity
Figure 1-57. A simple electrical resistivity measuring device, shown here, has a battery, a microampere meter, and wires attached to electrodes (nails). This test shows that a sample of bornite (copper "peacock ore") conducts electricity quite well. Parts of a flashlight can be used to make an electrical conductivity testing device.
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