HomePhysical 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.

 

1.1      Durability

1.1.1     Hardness

Hardness: "The ability a stone possesses to resist abrasion when a pointed fragment of another substance is drawn across its smooth surface without sufficient pressure to develop cleavage" (GIA course material).

 

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 or a hardness 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 jewellery, 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.

 

1.1.1.1   Significance of Hardness

To the lapidary:

Hardness is significant in choosing which grinding or polishin medium should be used with a particular material

To the gemologist:

  • Important factor in observation and testing

YET

  • the value of hardness to the gemologist is strictly limited
  • Information garnered by hardness tests is often of doubtful value

·       Any scratch detracts from the value of a gem. The danger of defacing or even breaking a valuable stone is much too great to permit common use.

·       It will not tell if something is synthetic or natural.

 

Hardness testing is not often used as the chance of damaging a good stone or even an imitation of value to the owner is too high. It is normally only used on rough material or on an inconspicuous spot on large carvings as a confirmatory test.

 

Hardness Plates Sheets or slabs of standard hardness materials. The gem to be tested is rubbed on the plate using the girdle so that hopefully the plate suffers the damage. Again, material can scratch itself although it is true that the feel of the "bite" in hardness testing can tell a great deal.

 

It is also not necessary to file chunks from gems or scratch whole facets; a 1 mm scratch can suffice and if the plate and stone is wiped clean and inspected with a loupe one can tell which was scratched. Diamond is the only colourless gemstone which will produce a scratch in a polished corundum plate.

 

Hardness Testing

·       Potentially destructive test

·       Use a steel probe or a set of hardness pencils set, with small sharp fragments of Mohs’ minerals.

·       Always work from soft to hard pencils so that only one scratch is obtained; that of the least hard pencil that is able to mark the mineral

Consideration

·       Using rough, a scratch can be attempted on any relatively smooth surface

·       Testing is done only with sharp-edged objects on fresh, non-decomposed crystals or cut surfaces (corrugated or foliated formations feign a lesser hardness)

·       Carry out tests discreetly in a place not normally visible and in such a way the tester need a magnifier or microscope to see the result of the test

·       Observe the process under a low-power microscope

·       This enables one to see a scratch a small fraction of an mm in length

·       After scratch is made, wipe surface clean and examine under microscope again to determine whether a scratch has indeed been made

·       Diamond can scratch diamond; do not use a diamond point to test a diamond.

·       All observations to be done in good lighting.

1.1.1.2   Cutting Resistance (Rosiwal)

Schumann: For the gemstone cutter, naturally the hardness of a stone, when cutting, plays an important role. There are also gemstones which are of a different hardness on different crystal faces and in different directions. For the stone collector, small differences in hardness are of lesser importance. In kyanite, for example the Mohs’ hardness along the stem-like crystals is 4.5, but across it is 6 to 7. There are also important differences in hardness in diamond on the crystal faces.

For the gemstone cutter, it would be helpful to have absolute values regarding the cutting hardness of gemstones. Unfortunately, there are hardly any useful numbers available. The cutter must rather discover for himself in practice and rely on his experience.

It is real art to cut softer gemstones, which only a few specialists master. If the crystal faces of a stone, in addition, are of different degrees of hardness, it takes a lot of skill to form sharp and even edges on these stones.

When polishing gemstones, the hardness is of utmost importance, because harder gemstones take a polish better than softer stones.

1.1.1.3   Mohs’ Scale(Scratch Hardness)

Mohs Scale

Other reference points include:

1. Talc

Finger nail 2 1/2

2. Gypsum

Copper penny 3 or so

3. Calcite

Window glass 5 1/2 or so

4. Fluorite

Knife blade 6

5. Apatite

Steel file 6 1/2 - 7

6. Orthoclase feldspar

Silicon carbide 9 1/4

7. Quartz

Carborundum 9 1/4

8. Topaz

 

9. Corundum

 

10. Diamond

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1.1.1.4   Knoop Indentation

1.1.1.5   Differential Hardness

  • Careful tests reveal that hardness of any crystal substance may vary on different crystal faces
  • Even the same face of a crystal may show a difference in hardness when scratched in different directions
  • Therefore, hardness is a directional property; hardness differs with the direction of scratching in certain materials

1.1.2     Toughness

1.1.2.1   Cohesion

1.1.2.2   Fracture

  • Fracture: Defines the type of surface obtained by breaking a crystal in a direction other than that of cleavage. Types include conchoidal, shell-like as in glass and often in gemstones. Also even, uneven and hackly or splintery as in nephrite. Identification applications of cleavage/fracture include: Nephrite cleavage cracks occur as 124o and jadeite at 93o.
  • Tulane lecture notes: If the mineral contains no planes of weakness, it will break along random directions called fracture. Several different kinds of fracture patterns are observed.
    • Conchoidal fracture - breaks along smooth curved surfaces.
    • Fibrous and splintery - similar to the way wood breaks.
    • Hackly - jagged fractures with sharp edges.
    • Uneven or Irregular - rough irregular surfaces.

 

1.1.2.3   Cleavage

  • Describes the tendency of a single crystal to beak along atomic planes. As atomic planes form crystal faces, cleavage always takes place parallel to a possible crystal face. (Hughes p.60)
  • The is the tendency of a crystallized mineral to break in definite directions related to the crystal structure producing relatively smooth cleavage break surfaces. Cleavage planes are always parallel to a particular cleavage face, i.e. diamond cleaves in any of the four directions parallel to the faces of the octahedron. Almost all crystals have a tendency to cleave. Those with the least tendency to cleave include garnets, quartz, spinel (natural), beryl and zircon. Gemstones with a strong tendency to cleave include diamond, fluorite, topaz, peridot, kunzite (spodumene), euclase, sphene, axinite, feldspars, synthetic spinel, dioptase and calcite.

 

  • Cleavage is described by the crystal face to which it is parallel; diamond has octahedral cleavage, topaz has basal (parallel to the base of the topaz crystal prism). The ease with which cleavage occurs and the resultant smoothness of the cleavage break is described as perfect in topaz, indistinct and difficult in beryl. Cleavage can be used in cutting diamonds and it should be noted that stones with a strong tendency to cleave can be easily cleaved in polishing and setting procedures.

 

1.1.2.4   Parting

  • The end result of parting is identical to cleavage – splitting along a plane of weakness. Like cleavage, this tends to produce a distinctive, step-like[1] fracture surface. However, parting is due to structural defects, rather then the basic structure design, as with cleavage. Thus the number of possible partings is limited to the number of defective planes present. (Hughes p.60)

1.1.2.5   Significance of Fracture, Cleavage and Parting

  • Properties of cleavage fracture and parting involves a crystal’s reaction to external pressure or force. This is intimately related to bonding and atomic structure[2]. (Hughes p.60)

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.

A 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 coloured 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.

1.1.2.6   Flaws

1.2      Density and Specific Gravity

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.

 

1.2.1     Definition and Significance

Significance

The specific gravities of the major gemstones seldom overlap, and therefore the accurate SG determination provides valuable information, vital to gem identification.

Advantage

·       Tests can be made with minor additions to a diamond balance or with an inexpensive set of heavy liquids, and so it is not a difficult test to perform for most jewellers.

Density

Definition: Ratio of the mass of any given substance to its volume. Alternatively, it is the mass per unit volume of any substance.

Specific Gravity

Definition: Ratio of the weight of a substance to the weight of an equal volume of water at 4 degrees celcius(the temperature at which the density of water is maximum).

Yet in practice, the variation of water's density at 4 deg C to that at rtp is so slight it is insignificant, and it need not be accounted for in gem ID.

1.2.2     Archimedes’ Principle

Definition: A wholly immersed body loses in weight the weight of the displaced fluid.

 

1.2.3     SG Determination

1.2.3.1   Hydrostatic weighing

1.2.3.1.1  Tolerances
1.2.3.1.2  Other Balances

·       Berman Balance

·       Jolly Balance

1.2.3.1.2.1     Direct Reading Balances

·       Westphal

·       Penfield

·       Hanneman: The Hanneman balance is extremely inexpensive ($10.00 US), fairly fast and easy to use. It is very accurate and can give accurate results on stones as small as .5 ct with care while other balances are not accurate below 3.0 cts.

 

1.2.3.1.3  Causes of Error in Hydrostatic SG Determination

Causes of Error in Hydrostatic SG Determination

 

1. Too small a specimen.

 

2. Surface tension of water. One drop of detergent liquid the size of a pin head destroys surface tension in 1 liter of water.

 

3. Failure to degas water. Use boiled, deionized water that has been sitting for a time.

 

4. Failure to record and compensate for temperature corrections when using liquids other than water.

 

5. Failure to multiply SG by density of liquid other than water.

 

To avoid problems of surface tension other liquids than water may be used. Examples are Carbon Tetrachloride, toluene and alcohol. Ethylene dibromide was used but is now considered too dangerous. Recommended is toluene or alcohol. Because temperature affects the SG of these liquids one must refer to the literature for each temperature change when using them. In practice it may be better to find the exact SG of the liquid you are using at the time of use and then refer to your SG and temperature correlations as you build them up with time. Such liquids are used for small stones to increase accuracy.

 

1.2.3.2   Pycnometer

1.2.3.3   Simple use of heavy liquids

1.2.3.3.1  Method

Various liquids and chemicals have a wide range of SGs. When a stone is placed in a liquid of the same SG it suspends, and stays where it was placed (or sinks or rises very slowly). If it has a lower SG (is 'lighter') than the liquid it floats. If it is denser (has a higher SG, is 'heavier') it sinks. Therefore, given a range of liquids of known SGs it is possible to estimate or even determine the SG of a stone by its behavior in the liquids.

 

Heavy liquids should not be used at all without a properly tested fume hood. Cleanliness and proper working methods are essential because they can cause severe health damage and can be absorbed through the skin and by breathing. They belong in a properly equipped gemological laboratory.

 

The Advantages of Heavy Liquids Include:

1. Speed. It is very fast. One usually begins with the densest liquid and goes to the next less dense until the stone sinks to the bottom. If you are lucky it suspends.

 

2. One can use them for stones under 3 ct. Stone size makes no difference.

 

3. As an ancillary method it may be very rapid to check or corroborate other tests.

 

4. Very quick for separating different types of similar appearing stones from the same package. For example checking beryl (emerald) one might make up a liquid with the density of 2.71 (Indicator is calcite).

 

Disadvantages:

1. The liquids used are really toxic, hazardous to your health, messy, smelly, poisonous and in some cases corrosive.

 

2. Cracked, flawed stones may give inaccurate readings.

 

3. Stones must be unmounted.

 

4. Porous stones may not be tested (opal, turquoise, organic gemstones).

 

5. The liquids attack many plastics.

 

There are three liquids recommended by gemology text books for general use.

Compound

Specific Gravity

Bromoform

2.88

Methylene Iodide(Diiodomethane)

3.33

Monobromonaphthalene

1.49

 

This is used to dilute and lower the SGs of the other two. Liddicoat suggests the use of toluene which is very flammable and evaporates faster than monobromonapthalene but is cheaper.

 

Liquid

SG

Indication

Bromoform diluted with monobromonapthalene

2.65

Quartz, feldspars, iolite float, most other stones sink.

Bromoform, undiluted

2.88

Beryl floats, other green blue stones sink

Methylene iodide diluted with monobromonapthalene

3.05

Tourmaline floats, nephrite floats, jadeite sinks

Methylene iodide undiluted

3.33

Jadeite, peridot suspend or float or sink very slowly, topaz sinks, tourmaline floats, etc

                                                                                   

There are also solutions made up with an SG of 3.52 (diamond indicator) and 4.00 (corundum indicator) using a liquid called Clerici solution which may be diluted with water.

 

Indicators of clear gemstones may be used as a check on the liquid SG before use as evaporation etc. can change the SG. Manufactured glass SG indicators in a wide range are also available. It should be noted that relative speed of sinking is a good indication of SG range. If a stone sinks very rapidly then it is a lot denser than the liquid, slowly it is similar. The degree to which it floats on a liquid can also tell something about its SG. High floating means it is a lot less dense, low floating similar, suspends - the same. Make sure to tap or dunk a stone with tweezers to ensure that surface tension is not holding it up.

 

Although Clerici solution offers many advantages to the gem tester it is no longer recommended as it is extremely toxic, corrosive and recently shown to be carcinogenic. It demonstrates an exact correlation between SG and RI as it is diluted. It may be diluted until a gemstone suspends and then a drop of it placed on a refractometer and the RI found. The one looks at a straight line graph and reads off the SG of the solution and the stone. To reconcentrate it water is simply allowed to evaporate. At full concentration its SG is 4.28. The less toxic liquids can also be used in the same way to obtain a correlation between SG and RI. However they are not easy to reconcentrate. There are also other methods of obtaining the SG of a stone using heavy liquids.

 

1.2.3.3.2  Care and Caution

1. Adequate ventilation is necessary.

 

2. All tweezers, stones, etc. in contact with the liquids must be very carefully cleaned; perhaps with toluene as a solvent; between liquids to prevent contamination or corrosion of tools. Clean them extra carefully after finishing.

 

3. They are poison. No food, smoking or drinking is allowed when using heavy liquids.

 

4. Hands must be washed right after use whether or not they were in contact with the liquids.

 

1.3      Streak

Streak is the colour produced by a fine powder of the mineral when scratched on a streak plate. Often it is different from the colour of the mineral in non-powdered form.

 

Streak is the true colour of a mineral in a powdered form, obtained by rubbing the specimen across an unglazed porcelain streak plate.

Crushing and powdering a mineral eliminates some of the effects of impurities and structural flaws, and is therefore more diagnostic for some minerals than their colour. Streak can be determined for any mineral by crushing it with a hammer, but it is more commonly (and less destructively) obtained by rubbing the mineral across the surface of a hard, unglazed porcelain material called a streak plate.

 

The colour of the powder left behind on the streak plate is the mineral's streak. The streak and colour of some minerals are the same. For others, the streak may be quite different from the colour, as for example the red-brown streak of hematite, often a gray to silver-gray mineral. The combination of lustre, colour, and streak may be enough to permit identification of the mineral.

 

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.

 

1.3.1     Streak Test

Streak tests are fun things to do. There are many surprises and you can use them to amaze your friends. It is also very easy and inexpensive.

All you do is to rub a piece of gem material on a tile and look at the colour of the streak it leaves. Here are the details on how to perform a streak test. First, go to a building supply store and buy a ceramic tile. Some tiles have rippled backs. You need to find one that has large flat areas on the back. We use ceramic tiles because they are hard enough that the gems will rub off on them.

Next, chose your gem materials. (Use rough pieces you have collected, not finished gems. See the warning below.) Rub the material firmly against the back of the tile; you cannot do this on the glazed or shiny side. Now look at the streak it left and see what colour it is. That is all there is to it!

Here are some of the results you can expect. Dark green malachite will leave a green streak. No surprise there. Brown tigers eye leave a streak that is almost the same colour as the stone. Again, this is no surprise. Next, test some lapis. This dark blue gem will leave a very light blue streak. Hematite is a dark gray, almost black when whole. Guess what colour its streak is? The answer is rust red. You have to see this to believe it! Pyrite, or fools gold, is another surprising stone. This brassy coloured mineral will leave a black streak on your tile!

Now, if your collection contains some pieces of rough amethyst, tourmaline, or other richly coloured crystals, give them a test. Almost all of your stones will leave a white or colourless powder.

When you know the streak colours of your gems, you can put on a magic show for your friends. Arrange your tests so the first stones have streaks close to their natural colour. Then move on to the coloured stones that leave a white streak. Leave the big surprises, pyrite and hematite, to the end.

 

Before testing your stones, ask your friends what colour they expect to see. They will usually be wrong and you will delight them with the "magic" results!

 

Warning! Streak testing is destructive and will damage the stones you are working with. Do not use your mother's, (or anyone else's,) gems. Only preform streak tests with pieces of rough or damaged gems.



[1]Terrace-like

[2]I prefer calling it crystal lattice, since not all structures are atomic in nature.

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