The mineral's composition and crystalline structure impart the various physical properties that characterize each specimen. Knowledge of the properties of gemstones is important for the gem cutter and setter, as well as to the consumer who can use that information to care for the gem.
A perfect crystal is bounded by plane faces which meet at angles specific for each kind of material (angle analysis can identify minerals). A crystal may be cleaved in directions related to the external form or to a possible crystal form for the mineral. Sometimes two distinct minerals can have the same chemical composition with their differing properties being due to their different crystal structure. Crystal structure affects mineral properties more than their chemical nature. Examples here include diamond (carbon, cubic) and graphite (carbon, hexagonal) and Calcite (trigonal) and aragonite (orthorhombic), both forms of calcium carbonate.
The specific gravity of a gemstone is the ratio of the weight of the material to the weight of the same volume of water at a temperature of 4 degrees Celsius. In general, minerals composed of heavy elements will have a higher specific gravity than those composed of lighter elements, although bonding and crystalline structure can also effect the specific gravity.
Also, the more closely packed the atoms, the stronger the bonding, and the higher the specific gravity. Heavier gemstones are usually harder as well. The range is from amber, which has a specific gravity of 1.08 and opal, with a specific gravity of 2.05, all the way up to corundum (sapphires and rubies) with a specific gravity of 3.99, Spessartite garnet, specific gravity of 4.15, marcasite, specific gravity of 4.9, and cuprite (s.g., 6.0) and Cassiterite (s.g., 6.9). Diamond is in the heavy mid-range, with a specific gravity of 3.52.
There are several ways to directly measure the specific gravity. To arrive at a relative measure of specific gravity, heavy liquids are used. Gems are placed in liquids of a known specific gravity. If the gem floats, its specific gravity is less than that of the liquid; if it sinks, the gem is heavier than the liquid; and if the gem remains suspended, it is very close to the liquid's known specific gravity. Another useful specific gravity liquid is saturated salt solution (SG = 1.08) which is used to separate amber from most plastic imitations. Amber will float and the plastic imitations will sink.
Note: There are drawbacks to these heavy liquids though. All of the heavy liquids used to determine specific gravity are poisonous and breathing the vapors is not advised. Also gems susceptible to chemical attack, such as amber or hematite, could be damaged using this suspension method.
The hardness of the mineral refers to its resistance to scratching and abrasion and also to the cutting resistance. The more resistant the surface is to scratching, the harder the mineral, and the stronger the bonding forces are holding the atoms together. Gemstones are often tested by using the Mohs' hardness scale to determine just how hard they are. The harder minerals are more durable in that they do not scratch easily and will hold up better in jewelry. This scale was devised by an Austrian, Friedrich Mohs, and runs from talc, the softest (H=1), and diamond, the hardest (H=10). Simply stated a harder mineral will scratch a softer one, and minerals of the same hardness will scratch each other. Gems with a hardness of 2 or less are considered soft; those with hardness 3 to 5 are called medium; gems with hardness of 6 and over are hard (Schumann, 1997, p. 19).
Only 10 or 12 of the major gemstones have the ideal hardness or hardness greater than 7. Quartz gemstones (citrine, amethyst, etc.) range in the 7's, topaz rates 8, and corundum (sapphires and rubies) are a 9 on the Mohs' hardness scale. This ideal hardness designation stems from the fact that quartz (H=7) is the most abundant mineral on Earth and present as tiny particles in the dust that settles on jewelry, which can lead to scratching and abrasion. Therefore, dust may dull the luster and polish of gems with hardness of 7 or less. Diamond registers a 10 and is the hardest known naturally occurring material on earth, more than ten times the hardness of corundum at 9.
Talc is the softest mineral with a hardness of 1 and can be easily scratched with a fingernail. There is more of a spread between the gems and minerals found between 2 and 3 and between 5 and 6, however corundum is only about 10 per cent harder than topaz. The hardness is relative, but it is, nevertheless, a useful identification tool. Hardness is almost never used as a separation test with gemstones since it is considered a destructive test and other nondestructive tests exist to enable separation and identification.
Hardness testing is acceptable with some rough material, but rarely done on fashioned gems. It is a test that is never used on transparent stones. It is a destructive test, which separates atoms and actually leaves a groove on the specimen.
Note: For the gem cutter, knowledge of hardness is important. Because hardness is related to bonding, different hardness can occur on the same gem in different directions, which means hardness can have an effect on durability as well as beauty. Harder minerals will result in sharper facet junctions and take a better surface polish.
Cleavage and Fracture:
Cleavage and fracture refer to the characteristic manner in which gems will break when an external force or stress is applied. Some minerals have a special way of breaking parallel along planes of atomic weakness, creating smooth flat surfaces. This break is called cleavage. Crystalline minerals have cleavage and fracture, whereas amorphous or massive stones only fracture.
In rough material, a cleavage break may already be obvious or it can be determined by giving the specimen a tap with a hammer. Rough diamond is often cleaved and then cut into shapes. Cleavage is not possible to observe in fashioned gems unless an internal imperfection can be observed or there is an accidental blow struck along a cleavage direction and the gem breaks. Thus, diamond has very well developed cleavage and although it is the hardest known substance, the ready cleavage makes it susceptible to damage.
Knowledge of cleavage for the cutter is important as it can lead to an easy first step to the fashioning process for diamonds. When considering colored stones, cleavage is avoided as it is very difficult to polish a gem parallel to a cleavage plane (Hurlbut and Kammerling, 1991, p. 54).
The heat produced when soldering the setting can cause fissures along cleavage planes and may lead to the gem actually breaking along these fissures (Schumann, 1997, p. 22). Piercings or drilling should be done vertically to the cleavage surfaces (Schumann, 1997, p. 22).
Fracture is the way a stone breaks. It is a break in a direction other than along cleavage planes and results when the bonding forces are similar in all directions. Consider fracture to be similar to a piece of wood breaking in a direction other than the direction of it's grain. A distinctive, common fracture is called conchoidal, which is a shell-like break. This break is seen in glass, quartz, opal, peridot, and amber, to name a few. Other possible fractures include uneven, splintery, granular, or subconchoidal.
Tenacity or Toughness:
Tenacity or toughness is the ability of a stone to withstand pressure or impact. It is the resistance to crushing, breaking, or tearing. Minerals which crumble into small pieces or a powder are said to be brittle. Tenacity terms include flexible, elastic, malleable, sectile, and ductile. If a gem bends but returns to its original position, it is said to be elastic (mica, nephrite, jadeite); these minerals are tough and difficult to break.
The jade gemstones (jadeite, nephrite) are the toughest of all gems, making them also difficult to cut. Talc and gypsum are examples of minerals which are flexible. Ductile or malleable minerals are those (gold, silver, etc.) which may be flattened out into thin sheets under pressure. The brittleness factor of a gemstone is an important consideration in gem cutting and polishing.
Many gem crystals shatter or chip easily, and this must be taken into consideration when cutting. Diamond is the hardest known substance but because of well developed cleavage and a brittle tenacity, it can easily shatter when hit.
The degree of tenacity:
Exceptional - like nephrite and jadeite jade
Excellent - like corundum
Good - like quartz
Fair - like tourmaline
Poor - like topaz
Note: A fair or poor tenacity does not mean the gem is less valuable, but does have implications for care and cleaning as well as setting the stone in a secure, protective mounting.
Magnetism and Electricity:
Those stones which are attracted by a magnet are considered magnetic, such as magnetite and hematite, which contain iron. Hematine, an imitation of hematite, is magnetic, whereas most natural hematite is very weakly magnetic. Synthetic diamond can contain iron-nickel flux inclusions and can show magnetism (when floating in a heavy liquid such as Clerici's solution), whereas natural diamond exhibits no magnetism.
The ability of a mineral to conduct electricity is referred to as electro conductivity. This property is mostly characteristic of minerals with metallic bonding, such as gold, silver, and copper. Minerals with partial metallic bonding are semiconductors of electricity. Most gem minerals lack metallic bonding and thus are nonconductors, with the exception of natural and synthetic blue diamonds that do conduct electricity. Blue diamonds that are colored by artificial irradiation are electrical insulators and can be separated from naturally colored and synthetic blue diamond with thermal inertia meters (electrical conductometers).
Piezoelectricity: Piezoelectricity, or pressure electricity, is found in minerals that have polar axes or lack a center of crystalline symmetry. The crystal axes have different properties at the opposite ends of the polar axis, and when pressure is exerted at these ends, electricity can flow creating opposite positive and negative ends. Quartz and tourmaline are piezoelectric. Thin slices of quartz oscillate when subjected to alternating current, controlling radio frequencies of electronic circuits for radios (since 1921) and watches (Hurlbut and Kammerling, 1991, p. 64). Tourmaline has been used in pressure gauges since 1945, when the blast pressure of the first atomic bomb was measured.
Pyroelectricity: Pyroelectricity, or heat electricity, occurs in minerals with polar axes or lack the center of crystalline symmetry. As a function of temperature, such as display lighting or heat in a display window with sun, positive and negative charges can build up in some gems. This means tourmaline can attract dust particles more easily when heated.
Frictional electricity: Frictional electricity, or an electrostatic charge created by rubbing, is common in many gems. The ability of the gem to attract light objects is dependent upon the charge and was probably first recognized in amber more than 2500 years ago. The Greek name for amber is "elektron," origin of our word electricity.
Some stones are good conductors of heat, such as quartz, which draws heat away from the body when held and thus feels cold to the touch. Heat is conducted differently in various minerals according to their crystal system. A poor thermal conductor, such as amber, feels warm to the touch because it does not conduct heat away from the body. The surface of a genuine gemstone will de-mist more rapidly than that of glass or an artificial stone.
Thermal conductivity should also be considered when cutting gemstones, as some stones will need a cooling-off period during the cutting. This is also used in Thermal Conductivity instruments to differentiate diamond which conducts heat very well from its simulants and imitations.
Some instruments use it to identify other gemstones but they are expensive and of value only when used with care and some gemological knowledge. The use of standard stones is suggested and drafts to be avoided as they can change the readings. At its simplest this is the temperature test using tongue or lips for glass and plastic.