Petrified tree trunk outside
Smithsonian. Photo by S.W. Aber,
4/2009, taken of Smithsonian
National Museum of Natural History
GO 340 Gemstones & Gemology
ES 567 Gemstones of the World
Dr. Susan Ward Aber,
Geologist & Gemologist
Emporia State University
Emporia, Kansas USA
Physical Properties 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.
The image below is quartz from Minas Gerais, Brazil. Quartz does not cleave but rather it fractures. A common fracture is conchoidal. Flat or rough crystal faces can be growth phenomenon, not an expression of fracture or
Photo by S.W. Aber, 4/2009.
National Museum of Natural History.
Pictures taken at Smithsonian
Cleavage and Fracture
Cleavage and fracture refer to the characteristic manner in which gems will break when an external force or stress is applied. The ease of breaking will effect the durability, an important attribute of gems. Some minerals have a special way of breaking parallel along planes of atomic weakness, creating smooth flat surfaces. This break is called cleavage. 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 suspectible 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 a break in a direction other than along cleavage planes and results when the bonding forces are similar in all directions. 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.
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. Hardness is measured on the Mohs Scale of Hardness. 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, which is greater than 7. 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.
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. For the gem cutter, a 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.
The tenacity or toughness of a mineral is the resistance to crushing, breaking, or tearing. This is another factor in the durability of the gem. Diamond is the hardest known substance but because of well developed cleavage and a brittle tenacity, it can easily shatter when hit. In contrast, nephrite jade has a hardness of 6, but is very tough because of the intergrown fibrous crystals. Tenacity terms include flexible, elastic, malleable, sectile, and ductile. The degree of tenacity is: "exceptional (e.g., nephrite and jadeite jade), excellent (e.g., corundum), good (e.g., quartz), fair (e.g., tourmaline), and poor (e.g., topaz)" (Hurlbut and Kammerling, 1991, p. 57). 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.
StabilityStability is the gems resistance to fading or other alteration due to light, heat, or chemical attack (Hurlbut and Kammerling, 1991, p. 57). Minerals classified as carbonates, such as malachite, pearl, and rhodochrosite, may be damaged by chemical attack. Dilute acids are actually used to positively identify some carbonates in mineralogy or to dissolve surrounding limestone to free a fossil! But in the gemstones mentioned or others such as onyx marble or onyx calcite (neither are onyx), care should be taken to avoid contact with acids, which includes some ordinary household and jewelry cleaners. Light and heat, from direct sunlight or artificial lighting of a display case, can damage gems as well. Kunzite, the pink variety of spodumene, and yellow-brown topaz can fade due to exposure to light, especially the strong light as in jewelry or museum displays. Gems that contain water, such as opal, may become dehydrated which can cause cracking or the loss of the play-of-colors phenomenon. Gems that are artificially irradiated can fade when exposed to light or heat as well.
Image above: Exceptionally dark kunzite
somewhat hidden in its mounting
and less direct lighting.
Image left: Kunzite rough and faceted
with bright display case lighting.
Photos by S.W. Aber, 4/2009, taken at
Smithsonian National Museum of Natural History
The specific gravity of a gemstone is the ratio of the weight of the material to the weight of the same volume of water. 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.
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.
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 suspectible to chemical attack, such as amber or hematite, could be damaged using this suspension method.
To summarize, go to:
Notes on specific gravity by C. Lewton-Brain and Ganoksin Jewelry Co., Ltd.,
Good sites showing how to determine specific gravity are online from Robert James, at http://www.yourgemologist.com/SpecificGravity/specificgravity.html, Understanding Specific Gravity.
Units of Weight
Units of weight used in international gem trade include carat, gram, and grain (Schumann, 1997, p. 26). Carat, with a c, is derived from the seed of the African Coraltree or Carob bean. In 1907 the U.S. adopted the metric carat of 200 milligrams or 0.2 gram. The carat can be divided into fractions or points. One hundred points is equal to one carat (e.g., one-quarter of one carat, or .25 carats, would be equal to 25 points). Karat, with a k, is a measure of quality, not weight, and used with metals and will be discussed in a later lecture. Gram is used for pricing rough stones and gems of lower values. Grain was used as a weight for pearls and corresponds to 0.05 grams or 0.25 carat.
To find out more about carat visit these outside links:
http://www.dendritics.com/scales/carat-def.htm, All About Carats from Dendritics (2003).
http://www.dendritics.com/scales/standard-formula.asp, Standard Gemstone Formula, which provides the formula to determine carat weight knowing length, width, and depth. This is from Dendritics (2003).
http://www.rockhounds.com/rockshop/gem_designs/gem_weight_form.shtml, an online gem weight calculator!
Streak Streak is the true color of a mineral in a powdered form, obtained by rubbing the specimen across an unglazed porcelain streak plate. This is a destructive test and is rarely used in gem identification. Hematite has a reddish-brown streak, whereas hematine, a common imitation of hematite, has a brownish-black steak.
Electrical and Magnetic
The ability of a mineral to conduct electricity is referred to as electroconductivity. 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, 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, 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, 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.
Minerals attracted to a magnet are magnetic. Most gemstones show no reaction with the exceptions of hematite-hematine and diamond-synthetic diamond. 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.
Thermal Reaction Tester and Thermal Inertia Meter A thermal reaction tester, or hot point, is used to test the gem's smell and reaction to high heat. Tortoise shell and some coral emit a burning hair smell. Jet, a fossilized coal, emits a petroleum aroma. Amber produces a resin smell and whitish smoke, whereas plastic imitations have an acrid, chemical odor. Wax and plastic treatments applied to gems can also be detected. The thermal inertia meter, or once referred to as thermal conductivity meters, measures how quickly the surface temperature of a gem changes when heat is applied. This test can separate diamond from its various imitators because the imitations have lower thermal inertias.
The material for this section came primarily from:
- Hurlbut, C. S., & Kammerling, R. C. (1991). Gemology. NY: John Wiley & Sons, Inc.
- Schumann, W. (1997). Gemstones of the world. NY: Sterling Publishing.
Return to the Syllabus or on to the next lecture.
This page originates from the Earth Science department for the use and benefit of students enrolled at Emporia State University. For more information contact the course instructor, S. W. Aber, e-mail: email@example.com Thanks for visiting! Webpage created: 1999; last update: August 30, 2012.
Copyright 1999-2012 Susan Ward Aber. All rights reserved.