Mineral Physical Properties

Introduction to Physical Properties

Physical properties are often the first tools used by mineralogists to identify a mineral in the field or lab. These properties use what we can see or observe as the starting point for classification. Such properties include Hardness (e.g., Mohs scale), luster (e.g., metallic, vitreous), color and streak, cleavage and fracture, and density (specific gravity).

The physical properties of a mineral reflect its internal atomic structure and chemical composition. For example,cleavage is determined by the arrangement of atoms and bonding strength in certain planes. Hardness reflects the bond strength between atoms in the crystal lattice.

The physical properties of minerals also determine their practical uses. For example, diamond's extreme hardness makes it ideal for cutting tools, while magnetic minerals like magnetite are used in ore exploration and industrial applications.

Furthermore, physical properties can provide information about the environment in which a mineral formed. Crystal habit and form can indicate the conditions of growth (e.g., temperature, pressure). Color or fluorescence can reveal trace elements and geological processes.

Finally, collectors often focus on physical properties like color and transparency, luminescence, and crystal habit for aesthetic or rarity considerations.

Let's explore these concepts in more depth.

Specific Gravity

Specific gravity (SG) or density refers to how much something weighs in relation to its size. For example, a cubic centimeter of steel is much heavier than a cubic centimeter of styrofoam. SG is expressed as the relationship to an equivalent volume of water. For example, a mineral with a specific gravity of 3 weighs three times as much as the same amount of water.

In mineralogy, the terms specific gravity and density are often used interchangeably, but there are some differences you should know:

Units

• Density has units (e.g., g/cm³).
• Specific gravity is unitless.

Measurement

• Density requires measuring mass and volume.
• Specific gravity is often determined by comparing weights in air and in water.

Applicability

• Density is a broader scientific term used across disciplines.
• Specific gravity is commonly used in mineralogy and gemology because it simplifies comparisons without needing units.

Specific Gravity Values of Popular Gemstones and Minerals

As you can see from the chart, few stones share the same SG range. This makes SG one of the most useful physical properties you can have for identification. However, measuring specific gravity is difficult and time consuming. Thus, you normally only conduct this test when necessary. (See "The Art and Science of Identifying Gemstones" for new identification procedures that don't typically require SG testing).

Using Specific Gravity to Estimate Mineral Weight

When you're buying and selling gems and minerals, knowing the SG of a gem can help you make the right choice.

For example, a one-carat opal with an SG of 2 would be much larger than a one-carat sapphire with the same shape. Why? Because sapphire has an SG of 4. Let's say you need to find a new stone for a jewelry setting. While settings are measured in millimeters, gems are often sold by weight. If you order a one carat sapphire to replace a one carat opal, you're in for a big surprise!

If you know your stones well, you can estimate their weight by sight based on their SG.

Hardness and Toughness

The physical properties of hardness and toughness are often misunderstood. Their scientific meanings differ from their everyday meanings.

Hardness

Scientifically, hardness is defined strictly as the ability to resist scratching. Mineralogists use the Mohs Scale to define mineral hardness. The scale runs from 1 to 10, with a specific mineral representing each number or unit. Diamond is the hardest at 10. Ruby comes in at 9, and so on. Each mineral on the scale will scratch softer stones, those below it. In turn, harder stones, those above it, will scratch it. Minerals of the same hardness won't scratch each other.

Hardness has an important effect on how well a gem will wear as jewelry. As a general rule (though exceptions abound), stones of hardness 7 or more will wear well. Since common dust consists largely of quartz (hardness 7), anything softer will eventually lose its polish. Even just wiping the dust off of a soft gem or mineral (below 7 in hardness) will cause fine scratches. Over time, these fine scratches accumulate and diminish the polish.

Toughness

Hardness has nothing to do with durability or toughness. Toughness or tenacity is defined as the ability to withstand being hit. For example, compare glass and wood. Scientifically, glass is much harder than wood. It will easily scratch wood. However, you can hit a wooden board quite hard with a hammer without breaking it, yet glass will shatter with the slightest impact. So, while harder, glass isn't nearly as tough as wood.

You may not find toughness listed among mineral physical properties in standard reference books. It has no standard measurement procedures or units beyond verbal descriptions. The International Gem Society (IGS) factors a stone's toughness into its wearability rating.

Cleavage

Mineral cleavage is a weak bond between the molecules of a mineral in certain planes in its crystal structure. You can best understand cleavage as a physical property of minerals by comparing a mineral to wood. Wood can split easily along the grain. However, splitting it across the grain is difficult. Some minerals also have a direction along which they can easily split. This is the cleavage plane.

While cleavage affects toughness, it has no relationship to hardness. Diamond is the hardest material known. However, since it has cleavage planes, you can split it with a piece of steel. In fact, diamonds can break during normal wear.

Cleavage fractures always run parallel to one plane of the original crystal's surface. They're always straight, flat, and difficult to see if they're inside the mineral. You can spot them by the rainbow of colors that comes off a flat plane when light hits it at the proper angle. Although hard to see when small, cleavage fractures are very important for identifying and valuing a mineral. Additionally, they indicate a weakness that could expand and ruin the mineral.

Cleavage Descriptions

Mineralogists describe cleavage in terms of how easily the material will part. The usual terms range from perfect, which separates very easily, to good, fair, and poor. You may come across other terms, like imperfect. However, these are used less frequently.

Another factor in descriptions is the number of cleavage planes. Some stones, like topaz, only have a single direction of cleavage. Since topaz's cleavage plane runs parallel to the base of the crystal, lapidaries usually cut it off at this axis. Thus, topaz is reasonably durable in spite of its cleavage plane. Other stones, like feldspar, have cleavage on every crystal face. This makes cutting and setting problematic.

Please note that current cleavage terminology is less than accurate. Take topaz, again. Because it's described as having perfect cleavage, faceters know it can be cut with minimum care. On the other hand, spodumene is an absolute monster to cut since it separates so easily. Still more difficult, diaspore comes in a class by itself. Yet, spodumene and diaspore are listed as having perfect cleavage, the same as topaz and diamond.

So, if you're judging how well a stone will wear, look deeper into the cleavage than the simple listings. Diamonds and topaz wear well. They're commonly used as ring stones. However, kunzite, or pink spodumene, and some other gems would make very risky ring stones.

Fractures

Mineralogists also describe how a mineral breaks in ways other than along cleavage directions. They call this fracture. (Cleavage fractures are always flat, which makes them easy to identify and distinguish from other fractures). Although not always easy to name, fractures may give you clues for identification. You can almost always find fractures on the girdle or culet of a gem with a high powered microscope. Of course, you'll have better luck finding them on rough material.

Fracture types include conchoidal, fibrous, splintery, granular, uneven, and hackly. The names are based on what the fractures resemble. The most common type of fracture is called conchoidal. You'll see these on everything from glass to ruby. Thus, they won't help much with identification. However, discovering one of the less common fracture types would be an important clue.

Keep in mind that fracture has nothing to do with the physical properties of hardness or toughness. Corundum is one of the most durable gems and it shares conchoidal fractures with glass, one of the most brittle.

You can easily spot most of these fractures.

Conchoidal

Typically, this fracture looks like an impression of a scallop shell.

If the fracture is less complete, you'll still see concentric banded lines in a curved section.

Fibrous

Tiger's eye gems have fibrous fractures.

Splintery

Jade, ivory, and petrified wood (of course) often have splintery fractures.

Be careful not to confuse fibrous and splintery. The difference is one of scale. Fibers are thin and fine, while splinters are usually thicker and coarser.

Granular

Granular fractures are common to lapis lazuli and maw-sit-sit.

Uneven

Uneven fractures are often seen in sodalite and coral.

Hackly

You'll find hackly (sharp and jagged) fractures more commonly in metallic minerals.

Parting

Like cleavage, parting occurs on a flat plane parallel to one of the crystal surfaces. Unlike cleavage, this results from twinning, where the crystal grew in overlapping layers. These layers have distinct thickness. Whereas a mineral with cleavage will have it in every specimen, parting only appears occasionally. It's not present in every sample of a mineral species. For example, you'll find parting in some star rubies but not all of them.

Star sapphires and rubies commonly contain parting. It's somewhat common in chrysoberyl and quartz. You'll usually see undercutting on the parting planes, as in the example pictured below. Undercutting represents a serious weakness in a gem and will greatly decrease its value and wearability.

Stability

Stability refers to a mineral's ability to remain the same. Although most minerals are very stable, don't take that for granted. A number of factors or sensitivities can affect the stability of certain minerals. You should consider sensitivities in general, and light sensitivity in particular, before you acquire a new mineral, especially one you haven't worked. Leaving your beautiful, valuable, new acquisitions on your desk or in a bright display case could cause them to change color or dry out.

Heat Sensitivity

The most common mineral stability problem is heat sensitivity. Actually, minerals don't mind getting hot. Rather, the rate of change affects them. Thermal shock usually occurs during the cutting or setting of a gem. Lapidaries and metalsmiths need special training on how to handle different gems to avoid thermal shock.

Opals are so heat sensitive they even need special care when worn. You may cause them to crack by walking from a heated room into the winter cold or from an air conditioned room into the full summer sun. Since opals contain water, drying can also cause them to crack. To avoid cracking, store opals in an air tight container or with a small piece of damp tissue. When wearing opals, keep them under your clothing if you're going between environments with extreme temperature variations.

Chemical Sensitivity

Certain chemicals can affect the look or composition of a mineral with chemical sensitivity. This poses a major problem for porous minerals. For example, turquoise and lapis lazuli can change color by absorbing oils from your skin. Therefore, don't wear these stones while working on your car, painting, gardening, or doing anything where you can come in contact with chemicals.

Certain acids can easily dissolve carbonates. These minerals include malachite, pearl, rhodochrosite, and marble. Be aware that many common household and jewelry cleaners contain these acids. Consult our gemstone jewelry cleaning guide for recommendations for caring for these and other stones.

Pearl is notoriously sensitive to chemicals. Always put on your pearl jewelry last when you're getting dressed. Hairspray and atomized perfumes can ruin them.

Light Sensitivity

The colors of some gems will fade if exposed to enough light. Such light sensitive gems are called "evening stones," since they're best reserved for evening wear when they're not exposed to sunlight. Even display case lights can damage light sensitive gems. Thus, be careful when displaying these gems for sale. (In addition, display lights can produce enough heat to dehydrate opals). Many gems suffer from light sensitivity to some degree. Some brown and gold topaz will fade slowly over time. As kunzite loses its color, it also loses a lot of value.

Even within the same species, sensitivity can vary. Blue spodumene, for example, is somewhat light sensitive. However, hiddenite (green spodumene) is so light sensitive you should keep these rare specimens covered and only take them out for brief periods.

Artificially irradiated gems and minerals can also fade when exposed to light or heat.

Streak

Optical properties largely determine a mineral's color. Streak, however, is one of the physical properties. It shows the color of a mineral without selective absorption in play. To conduct a streak test, you powder a tiny bit of a stone by rubbing the specimen across an unglazed porcelain tile or streak plate. Some stones have a streak color that differs considerably from what it shows under light. For example, most transparent colored stones, including emerald, will show a colorless or white streak.

Streak testing can help identify some minerals. Hematite, an opaque metallic gemstone, has a reddish-brown streak, whereas hematine, a common hematite imitation, has a brownish-black steak. However, don't conduct this destructive test on finished stones. Test material in inconspicuous spots as a last resort only.

Magnetism

Magnetism is one of the most misunderstood physical properties. The terms magnetism and magnetic refer to things possessing a magnetic field. Mineralogists don't use these terms to describe things that are attracted to magnets. Rather, they describe things that are magnets themselves. For example, magnets attract ferrous metals. Hematite, a form of iron ore, would be attracted to a magnet. However, hematite itself is not magnetic, while hematine is. It will pick up metal objects.

Tests for identifying magnetism usually involve suspending the gem from a thread and holding it near a magnet. Although they work, you'll put in a lot of labor for the little information you'll receive. Keep this property in mind, but know there are other, much easier and common gem identification tests.

Electrical Properties

Minerals have a variety of electrical properties that become important for industrial applications.

Electroconductivity

As the name implies, this is the ability of a material to conduct electricity. Minerals with metallic bonding, such as gold, silver, and copper, commonly possess this ability. A few minerals with partial metallic bonding are electrical semiconductors.

Most minerals are nonconductors of electricity because they do not have free-moving electrons or ions that can carry an electrical current. However, there are exceptions:

• Conductive minerals: Some minerals, like graphite (a form of carbon) and pyrite (iron sulfide), can conduct electricity due to their ability to allow electrons or ions to move freely through their structures.
• Semiconductors: Minerals like silicon and germanium are semiconductors, meaning they can conduct electricity under certain conditions (like when doped with specific impurities).

In general, most minerals, especially those with covalent or ionic bonds, do not conduct electricity in their solid state, making them nonconductors.

Piezoelectricity

Some minerals, like colemanite, quartz, and tourmaline, generate electricity when placed under pressure. This property, piezoelectricity, is common in several minerals to various degrees. When pressure is exerted at the ends, electricity will flow and create opposite positive and negative poles. Due to this property, quartz and tourmaline have various industrial applications. For example, thin slices of quartz can control frequencies for radios and watches. Some pressure gauges utilize tourmaline. In fact, tourmaline was used to measure the blast pressure of the first atomic bomb in 1945.

Pyroelectricity

Some minerals generate an electrical current when heated. Examples include boracite, colemanite (also piezoelectric), rhodizite, and sphalerite.

Frictional Electricity

When rubbed, many minerals commonly develop an electrostatic charge or frictional electricity. Notable examples include tourmaline and amber. If you rub them against something like wool, these gems gain enough charge to pick up ashes or small pieces of paper. The Ancient Greeks recognized this quality in amber over 2,500 years ago. The word "electricity" comes from the Greek name for amber, elektron.

Thermal Conductivity

Some stones, notably crystalline minerals, make good heat conductors. Due to their thermal conductivity, they will draw heat away from your fingers. Thus, they feel cool to the touch. Poor thermal conductors, such as amber, glass, and plastic, feel warm to the touch. They don't conduct heat away from the body. If you pay attention to how a stone feels in your hand, you can often spot an imitation. When in doubt, use a sensitive part of the body like your tongue or lips (but only if you've recently cleaned the stone). Crystalline mineral surfaces will also de-mist more rapidly than amorphous glass.

Scientists use thermal conductivity instruments to differentiate diamond, which conducts heat very well, from its simulants and imitations. Avoid drafts when testing, as they can change the readings. Additionally, thermal inertia meters measure how quickly the surface temperature of a mineral changes when heated. This test can also help identify diamonds, since they have much higher thermal inertia than other minerals.

Hot Point Testing

Thermal reaction testing, or hot point testing, can also distinguish real minerals from imitations. You can use a pin heated over a flame to generate smoke and odors from gem samples and show reactions to high heat, such as melting or bubbling. Since melting is common to plastic, amber, wax, and resins, that result means your sample either is one of those materials or was surface treated with them.

Odors can often be diagnostic. Tortoise shell and some coral emit a burning hair smell, while jet, a fossilized coal, has a petroleum aroma. Amber produces a resinous smell and whitish smoke. Plastic imitations have an acrid, chemical odor. Odors can also reveal wax and plastic surface treatments applied to minerals. For example, turquoise should have no smell. However, you may detect the odor of any treatment on the surface of the mineral.

Hot point testing is another destructive test. Test material in inconspicuous spots as a last resort only.

Mineral Physical Properties Self Test

Review your knowledge of physical properties with this little true-or-false test.

1. Density and specific gravity are the same thing.

2. Hardness is a clear determination of a stone's durability.

3. Cleavage fractures are shell-like.

4. Knowing the type of fracture in a mineral will tell you how durable or brittle it is.

5. Parting is a result of twinning.

6. Evening stones are sensitive to heat.

7. Hematite is magnetic.

8. Thermal conductivity tests are best preformed with your tongue or lips.

Editor's Note: The original article has been updated and adapted for the Mineralogy Certification course.

Donald Clark, CSM IMG

The late Donald Clark, CSM founded the International Gem Society in 1998. Donald started in the gem and jewelry industry in 1976. He received his formal gemology training from the Gemological Institute of America (GIA) and the American Society of Gemcutters (ASG). The letters “CSM” after his name stood for Certified Supreme Master Gemcutter, a designation of Wykoff’s ASG which has often been referred to as the doctorate of gem cutting. The American Society of Gemcutters only had 54 people reach this level. Along with dozens of articles for leading trade magazines, Donald authored the book “Modern Faceting, the Easy Way.”

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