Cleavage: In the three dimensional structure of certain crystals, atoms are bound more tightly to each other in some directions and more loosely in others. As a consequence, when strong forces are applied, relatively clean breaks may occur in these "weakest link" directions. These breaks, which can sometimes be so smooth as to appear to have been polished, are called cleavages. The number of directions in which a particular material cleaves, the ease with which that happens, and the "perfection" of the breaks are used to quantify this characteristic.

Since cleavage, or lack of it, is a species trait, it also serves as a good gem identification criterion. In the examples below, the number and completeness of cleavage of three species are shown.

Species with easy or perfect cleavage, particularly when such is the case in multiple directions, are poor risks for most jewelry applications. Not all gems show cleavage however, for example tourmalines, sapphires, and garnets do not.

[Apatite: two, imperfect (note that cleaved surfaces are somewhat rounded and irregular); spodumene锂辉石: two, perfect (note extremely flat, smooth breaks), fluorite: four, perfect]

Food for thought: Far from being a matter limited to academic interest, knowledge of gem cleavage has practical value, both as a means of gem identification, and in the appropriate fashioning and selection of gems for a particular use. (Answers to the questions below are found at the end of the lesson).

Question 1: Suppose you're a budding gem cutter or collector, and you happen to be at a swapmeet where a vendor has some transparent pink gem rough to sell. He knows that it is either Kunzite (pink spodumene) or pink tourmaline, but just can't remember which one. You have been wanting some pink tourmaline, so you look at the material closely and can't find any evidence of cleavages, even using your 10 power magnifier. Of the two choices, which is it most likely to be?

Question 2: A big decision is coming up in your life--> you are about to choose an engagement ring. Not being a slave to tradition, you are considering a colored stone for the piece, rather than a diamond. You want a blue stone, and your top contenders are: blue topaz, and blue sapphire. Considering that engagement rings are worn all day, every day, for many years, you do not want a stone that is likely suffer a cleaveage that will crack or break it. Which is your best choice? (Hint: look up topaz in the Lyman text pg. 128).

Question 3: You've found a beautiful piece of apatite rough and want to have a stone cut from it . You approach your friend who is a facetor, and ask him/her to cut you a marquis shaped stone from the piece. The cutter declines and says they will cut an oval or round but not a marquis. Why?

Miners have long used the cleavage properties of gems in trimming the stones they find. "Cobbing" is the act of smacking a piece of rough sharply and precisely with a hammer to break off any unstable (already partially cleaved), or included areas. Knowledge of the cleavage planes in the material being mined is essential to efficient use of this technique.

The use of cleavage is perhaps most well known in diamond cutting. We've all seen photos or videos of that tense moment when the diamond cutter inserts the wedge at a particular spot on the diamond and strikes it with a mallet. If all goes well, the stone splits precisely where the cutter wanted, and expected, it to. It is said that the expert that first cleaved the (up to that time) largest rough diamond ever found (The Cullinan) had studied it for months to determine its cleavage planes, and upon striking the first blow fainted dead away from anxiety. All was well, however.

**Check the text: See page 7 in the Hall text to view the largest of the many cut stones from the Cullinan, in its home in the Royal Scepter of the British Crown Jewels).

Fracture: Whereas cleavages occur only in some gems, and within those, only in certain directions, fractures can, and do, occur in all gems, and in any direction. A fracture is a break which is not along a cleavage plane. With sufficient force, any gem will fracture, although some do so more readily than others. The edges of fractures are not smooth like those of cleavages, but they do tend to have one of several basic appearances.

Playing on the resemblances of certain fracture types to well known surfaces and objects, terms like conchoidal (shell-like), splintery, uneven, step-like, and granular are used. Like cleavage, this is a species specific characteristic which has value in the identification of gems.

[Citrine quartz: conchoidal, Charoite: splintery]

[Turquoise: granular, coral: uneven] 

Conchoidal fracture is the most common, and is found in corundum, beryls, all the quartzes, opals, and both natural and man-made glasses. Turquoise and coral are commonly simulated by glass.... So:

Question 4: You are offered a bag of cut turquoise or coral at a gem show. The color is lovely and the price is very tempting. You notice that one of the pieces in the parcel has a broken edge which you examine with your 10x magnifier. With no knowledge other than what you've learned about fracture, what might you see on that broken edge that would tell you that this was not real turquoise or coral after all?

Durability Factors

In Lesson One, the concept of gem durability was introduced and described as being made up of hardness, toughness and stability. Let's now look at each of these factors in more detail.

Hardness: The tendency to resist scratching in a gem is known as its hardness. Of the three factors comprising durability, it is the most familiar. Even those folks with just a passing interest in gems know that they can be ranked on a scale of hardness. Hardness is primarily the result of the strength of the chemical bonds between the gem's constituent atoms (how tightly they are bound to one another).

The hardness of a gem affects its wearability, luster, and resistance to cutting and polishing. All other factors being equal, harder gems are more useable in jewelry, develop a brighter surface luster, and take more time and effort to cut and polish. They will retain their polish longer than softer gems, given equal wear and tear.

The familiar 1-10 Mohs' Scale of hardness, is not an absolute measure, but rather a relative one ---> a kind of "pecking order". Gems ranked at a higher number on the scale can scratch those ranked lower, and will in turn, be scratched by those whose number is higher than theirs.

Frederich Mohs, a 19th century German mineralogist was the originator, and we still use his scale, with the minerals which he designated as reference points today. For example, (softest) talc = 1, quartz = 7 and diamond = 10 (hardest).

[Talc, the softest on the Mohs' scale, diamond, the hardest] 

In mineralogy, one of the key tests commonly used for purposes of identification is a "scratch" test, which is done with a set of implements known as hardness points. These, usually steel, "pencils" are tipped 镶嵌with various minerals (or metals) of known hardness. By drawing them across the surface of an unknown mineral sequentially, the tester can determine the sample's approximate hardness. In gemology, such tests are rarely used as they are destructive in nature. Exceptions might be in testing the bottom of a carving, or a piece of gem rough, or a bit of material which has broken off. Another drawback of the standard hardness points is that they are not precise, but limited to giving a "ballpark"大约的 estimate.

In a laboratory setting, exquisitely precise measurements can be made with sclerometers硬度计. These devices use diamond-tipped, hydraulically operated probes, and can give an absolute reading on the force necessary to penetrate the surface of a material.

Not many hikers背包客, nature lovers, or rockhounds奇石采集者 carry hardness points with around with them on their treks, but the use of just a few ordinary materials can allow such individuals to do pretty good hardness tests in the field.

The Practical or Field Mohs' Scale

1-2: easily scratched by fingernail

3-4: scratched by copper coin

5-6: easily, and not so easily, scratched with pocket knife

7: scratches window glass/scratched by steel file

8-10: scratches window glass, but not scratched by steel file:

Hardness can be directional. This is actually quite understandable, as it depends on chemical bonds which can differ in strength, and in distance from each other, depending on which axis of the crystal we are observing. Generally such differences are relatively small and of litttle consequence, but there are two notable cases where they are dramatic and important. 1) Kyanite is notoriously difficult to cut because of its extreme directional hardness differences. 2) Diamond cutting would scarcely be possible unless the cutters could use the directional hardness of that gem to their advantage (More about diamond cutting to come in Lesson 7).

SOFT GEMS:

[Ivory and jet: 2.5, pearl: 3, sphalerite: 3.5, fluorite: 4] 

GEMS OF INTERMEDIATE HARDNESS

[Scapolite: 6, Tanzanite: 6.5; garnet: 7 - 7.5 depending on species, tourmaline: 7.5]

HARD GEMS

[Spinel & topaz: 8; chrysoberyl: 8.5, sapphire: 9] 

Toughness: The tendency to resist breaking and chipping is known as a gem's toughness. This property is controlled primarily by two factors: the readiness of a material to cleave in single crystal gems, and the presence or absense of certain structural characteristics in aggregate and/or amorphous gems which promote strength and cohesion.

All other factors being equal, the harder the gem, the tougher it will be, but all other factors are not always equal. Take the case of topaz, for example. At hardness 8 it seems to be a pretty rugged gem, but if we consider its strong tendency to cleave in one direction, in reality, it is rather fragile.

Likewise, diamond, the "star" of the hardness game, is only ranked as "good" when it comes to toughness because of its cleavage and fracture potential. Diamonds are usually cut with a flat culet facet at the tip of their pavilion, rather than coming to a sharp point as do colored stones. This is due to the likelihood of a fracture (or cleavage) in the fragile culet zone. When purchasing a diamond it is a good idea to check the girdle under magnification to make sure that it is not excessively thin, as this is another site of special vulnerability. Likewise, the corners and points of cuts like baguettes法式长棍面包, trillions and marquis are vulnerable, and should be protected by the mounting when used in jewelry.

[Fracture on the girdle of a diamond: Image courtesy of Martin Fuller, cleavages on diamond with classic "staircase楼梯" pattern]

On the other hand, nephrite jade with its hardness of 6.5 might seem to be delicate, but due to the felted粘结起来的, fibrous nature of its aggregate crystals, it is literally the toughest gem on Earth! So it is with pearls, which with their extremely low hardness, would barely be wearable at all, except for their moderately good toughness. It results from the layered, overlapping nature of the aragonite 结晶碳酸钙，霰石mineral plates of which the pearls are made, and the proteinaceous . 蛋白质的 "mortar" 迫击炮；臼，研钵；灰浆 that holds these brick-like layers together.

Check the Web: In this short article with interesting micro-photos, researchers summarize recent advances in materials science whereby they attempt to make an artificial material with the structure and toughness of Mother of Pearl (nacre) that might be used, among other things, for bone grafts. 

Toughness affects both wearability and resistance to polishing. Jade gems thousands of years old are as beautiful today as when they were first made. A well polished jade is a sign of a dedicated and skillful lapidary, 宝石的；[宝] 宝石雕刻的  as its structural characteristics make it susceptible to "undercutting" and an "orange peel"皮  surface effect if not handled expertly and with patience.

There is no numeric scale on which toughness is measured, rather, relative terms such as: exceptional, excellent, good, fair and poor are used.

FRAGILE GEMS

[Topaz, sunstone, sodalite 方钠石 , serpentine: all poor] 

GEMS OF INTERMEDIATE TOUGHNESS

[Tourmaline, iolite: fair; chrysoprase (quartz), diamond: good]

TOUGH GEMS

[Sapphire, hematite: excellent; jadeite jade, nephrite jade: exceptional]

Stability: Stability in a gem is a measure of its ability to resist changes due to exposure to light, heat and/or chemicals. Not only does stability affect wearability, but it also dictates指示 appropriate ways of fashioning, cleaning and storing the gems. Most gems are stable, but a few (even some quite popular ones) are unstable, and must be handled accordingly.

The Effects of Heat

Dehydration: Heat is a factor that can create problems with certain gems. In some cases, the mineral comprising the gem is "hydrated", that is, it contains water molecules which adhere chemically with varying degrees of tenacity. When the water is rather loosely attached, hot dry air can lead to loss of some of the water, and changes in the color, or transparency of the gem. Even more seriously, its loss can cause a network of cracks to form in the gem, in a process called "crazing". Opal is the most well known gem for which this is an issue.

[Badly crazed opal: Image courtesy of Bill Wise] 

It is sometimes suggested that opal gems and jewelry items be stored in water or oil when not being worn --> this is NOT good advice. Water will not hurt the opal, but it will not help it either. The type of "chemically linked" water that is lost when crazing occurs cannot be replaced by soaking, nor can this procedure be used as a preventative.

It is the structural details of the particular type of opal, including the percentage of water in it, that determine the likelihood of crazing. Reputable opal dealers "proof" their material before it is sold, by subjecting it to hot dry conditions for months. Generally, those pieces that survive such treatment will be stable under normal wearing conditions.

(Leaving an opal on a car dashboard 汽车等的仪表板 for hours in the August sun, or forgetting that your opal ring is in your pants pocket, and then putting the pants in the dryer for an hour on high, would NOT be examples of normal wearing conditions! Soaking in oil is an especially bad idea as opal is a porous gem and the oil seeps inside and then discolors over time, degrading the gem's beauty. )

Thermal Expansion: Another problem that heat creates for some gems is caused by their inherent capacity for "thermal expansion". This is a yet another physical characteristic by which gems differ. Diamond is notably stable to temperature changes (with slow and even rates of thermal expansion), so much so, that jewelers can pour molten metal into molds containing wax models with the diamonds already in place, to cast pre-set jewelry pieces.

Other gems, such as apatite, expand so rapidly with sharp rise in temperature, that their crystal structure is damaged, and they crack or even shatter. Heat sensitivity of that degree makes it very important for lapidaries cutting such gems, and jewelers working on mountings containing them, to keep the gem cool during these processes.

The Effect of Inclusions: Although a gem might be quite temperature stable itself, inclusions of other minerals within it, could have different degrees of thermal expansion from their host. This situation becomes quite important in the heat treatment processes used to enhance gems. Internal inclusions can literally explode or, less dramatically, expand, and in doing so, create internal "stress cracks" in the gem being treated. (For this reason, it is standard practice among Tanzanite heat treaters to heat only cut stones which have had virtually all the inclusions removed, and to avoid heating rough material.)

To an extent, heat treaters can ameliorate改善such effects by very, very, slowly raising and lowering temperatures. Tanzanite heaters might take 12 to 24 hours to incrementally reach the desired temperature, hold the gems there for several hours, and then take another 12 to 24 hours to gradually cool them down. At the highest temperature levels, though, such as those required to heat treat corundum, or those used for "color diffusion" processes, nothing can prevent heat damage. This is good news in a sense, though, because such internal and external cues to the heating, help the jeweler or gemologist spot the gem as one which has been subjected to extreme temperatures.

[In the center of this picture of the interior of a gem under high magnification, you see an included, heat shattered crystal, broken into four pieces, and a series of stress fractures surrounding it--> positive evidence of high heat treatment in this gem]

There are cases where thermal expansion characteristics of gems are used to deliberately induce cracks or stress fractures. Pieces of amber which have been heated, and then quickly cooled, develop disk-like stress fractures called "sun spangles" 亮片；闪烁发光  which some consider to be attractive.

["Sun Spangles", stress fractures in heated amber] 

A very old method of dyeing gems, which is still occasionally used today, is called "quench crackling淬火的爆裂声 "--> single crystal gems, like quartz, for example, which would ordinarily not absorb dye are heated and plunged in cold water to fill them with cracks that, then, can take up the dye, giving apparent color to the whole piece.

[Quench-crackled quartz pebble dyed pink, the closeup shows clearly that the pink dye is confined just to the cracks]

Other Environmental Factors:

Light: Some gems can fade or change color when exposed to light. An extreme example of this phenomenon is seen in the rare mineral pyrargyrite深红银矿  which must be kept constantly under opaque covers or else light exposure quickly renders its originally red color completely black. In the case of gem minerals, there are only a few to be concerned about. Kunzite (pink spodumene) can lighten in color with long term exposure to bright light, and is sometimes suggested as an "evening only" gem. Certain brown topazes, notably those from Mexico, can lighten dramatically, even becoming colorless with continuous light exposure.

Chemicals: Exposure to various chemicals can ruin the polish of, and/or discolor certain gems. Two important cases would be carbonate gems, like rhodocrosite菱锰矿 , which degrade due to a chemical reaction when exposed to acids, and amber which can be dissolved by acetone丙酮 . It is doubtful that a drop of lemonade 柠檬水 , or vinagrette salad 醋汁沙拉  dressing, or a bit of spilled nail polish remover would harm such stones, but acid vapors found in the polluted air of many cities can take their toll over time, as can some intense solvents, such as paint strippers, which might be used in the home or workplace. A dip in certain jewelers' solutions, like the hot "pickle" used to remove oxidation from metals, would be devastating to rhodocrosite, while a few hours spent soaking in "Attack^TM" (a solvent used to remove glues used in jewelry making) would ruin an amber gem.

Most gems in the unstable category, however; are sensitive more in virtue of their porosity, than because of their chemical makeup. Pearls and turquoise are two gems well known for their propensity to absorb cosmetics, perfumes, body oils, sweat, etc., and to dull and discolor as a result. Often fine turquoise gems are given a final polish with a layer of colorless paraffin石蜡  wax to help seal and protect them from such degradation.

Lightly wiping chemically sensitive gems with a damp cloth after each wearing will help to keep them in good shape. Any gem which is suspected, or known, to be chemical or heat sensitive should be protected from steam or solvent cleaning methods. Such considerations also become a factor in gemological testing in that, turquoise, for example, cannot be placed in the chemicals that would be used to determine specific gravity, or those used in relative refractive index testing.

UNSTABLE GEMS

[Apatite and opal: heat sensitive, Mexican brown topaz: fades in light, turquoise: porous and likely to discolor with exposure to various materials]

Specific Gravity

Specific gravity, also known as relative density, differs widely among gemstones, and is one of their most important physical characteristics from the viewpoint of gem identification. Specific gravity (SG) is the ratio of the weight of one unit volume of the gem to the weight of the same unit of water. For example, to say sapphire (corundum) has SG = 4.0, means precisely that a cubic inch of sapphire weighs four times as much as a cubic inch of water. In natural gems, SG values range from just over 1 (1.08 for amber) to just short of 7 (6.95 for cassiterite).

LIGHT GEMS: SG < 3.O

[Amber: 1.08;, shell: 1.30, meerschaum:海泡石  1.50, opal: 2.10] 

MEDIUM DENSITY GEMS: SG: 3 - 4

[Andalusite: 3.16, jadeite: 3.33, chrysoberyl: 3.71, sapphire: 4.00]

HEAVY GEMS: SG > 4

[Zircon: 4.69, scheelite: 6.10, anglesite: 铅矾，[矿物] 硫酸铅矿  6.35, cassiterite: 6.95]

A curious student might ask at this point: "Why do specific gravities differ so much?" The answer, satisfyingly, goes back to the basic premise of this lesson (that all physical properties of gems are the result of their chemical and structural makeup).

The various elements of which gems are made have atoms of different weights. Atoms of gaseous elements like hydrogen and oxygen are light, while metallic elements like aluminum and iron have heavy atoms. Chemists use "atomic weights" to describe elements --> rounding them off, here are some examples: hydrogen = 1, carbon = 12, oxygen = 16, aluminum = 27, silicon = 28, calcium = 40, iron = 56, zinc = 65, and lead = 207.

It's quite logical, then, that a cubic inch block of lead is going to weigh much more than a cubic inch block of aluminum. Extending that idea to gems, we can see that if a gem is made of relatively heavy elements it will have a greater SG than if it is made of lighter ones.

There is a second factor to consider, however; which is the structure: How are those atoms put together? Are they tightly packed or loosely arrayed? The examples below will help to illustrate the interplay of chemical make-up and crystal structure in determining specific gravity.

1) First, let's look at the case where structure is held constant but atomic makeup is different. Here, we'll compare two minerals that have the same crystal structure, in this case they both are of the orthorhombic system, and, they have identical chemical formulas except for substitution of one element for another.

Calcite: CaCO₃ -vs- Smithsonite: ZnCO₃

Both consist of five atoms per unit: either a Ca or a Zn plus one carbon and three oxygens. Both are put together with the "atomic packing" characteristic of the orthorhombic system of crystals. Their SGs differ though, with calcite = 2.71 and Smithsonite = 4.35.

Looking at the list above and seeing that calcium's atomic weight is 40 and that of zinc is 65 gives us our answer!

Question 1: Suppose we had a 6 mm round calcite, and a 6 mm round Smithsonite, cut to the same proportions--> Which would be heavier? Or to turn it around, if we had a one carat round calcite and a one carat round Smithsonite, which would be bigger?

2) Now to examine the effect of structure, by holding the chemical makeup constant... Remembering the concept of "polymorphs" from the first part of this lesson, we'll compare calcite and aragonite. Both have the same chemical formula, CaCO₃:

Calcite: orthorhombic crystal system -vs- aragonite: trigonal crystal system

Both are made up of the same elements in the same proportions, but those building blocks are put together differently so their SGs differ, with calcite = 2.71 and aragonite = 2.94

Question 2: Suppose we had a 6 mm round calcite, and a 6 mm round aragonite, cut to the same proportions--> Which would be heavier? Or to turn it around, if we had a one carat round calcite and a one round carat aragonite: which would be bigger?

Question 3: Look up the SGs for gold and platinum in the back of the Hall text. (Even if platinum sold for the same price as gold, which it doesn't) why would it cost more to make a particular size and type of ring in platinum than in gold?

Measuring Specific Gravity: 

Although SG measurements can be made on either rough or cut gems, the gems must be unmounted, and composed of a single material. You cannot do a SG measurement on a gem that is set in a piece of jewelry, or on an assembled stone, like a doublet. Porous gems cannot be measured with at least two of the techniques, as the liquid they absorb affects the SG measurement, and, in some cases, can harm the stone. Detailed reference books meant for mineralogists or gemologists will list SGs for gems as a range, rather than a single number, due to the fact that individual specimens will differ slightly based on the number and type of their inclusions. (Your texts, meant for non-professional use, however, use a single number average of the SG range for the gem species). There are several ways in which SG is measured, and they differ in precision as well as suitability to different gems and circumstances.

Hefting: 面包酵母的发面力 The crudest technique, but one that can be rather useful in some situations, is simple hefting. By lifting the gem and gently throwing it up in the air and catching it, a general feel for its density can be gained. This technique is often all that is needed to discriminate plastic and some glass imitations from the much denser gems they mimic. Conversely, jewelers who are intimately familiar with the heft of a 6.5 mm diamond (which will weigh almost exactly one carat) may be able to quickly pick out a 6.5 mm imposter because so many diamond simulants have SGs substantially higher or lower than diamond.

Heavy Liquids: For most of us, though, in most circumstances, hefting would not supply enough information. One popular method is based on the principle of bouyancy: "an object will sink in a fluid of lesser SG, remain suspended in one of equal SG, and float in one of higher SG." This technique uses a set of "heavy" liquids with known SGs. By immersing the unknown gem material in the liquids, and observing its behavior, its approximate SG can be deduced.

[Heavy Liquids Testing Set, SGs of liquids are printed on bottles, dropper bottles are for calibration]

To give a simple example, consider an unknown gem that floats quickly in the 3.05 bottle, sinks rapidly in th 2.57 bottle, and floats and sinks very slowly in the 2.67 and 2.62 bottles, respectively. That would tell you that the SG was between 2.67 and 2.62 and would allow you to rule out a great many minerals and focus any further tests on a smaller group of "possibles". Corundum (SG = 4.0) would behave quite differently from these observations, and could be excluded, while quartz, whose SG is 2.65 would behave precisely as described, and could not, therefore, be excluded.

Hydrostatic Weighing: By far the most precise technique for SG determination involves use of a specially modified weighing balance that allows a gem sample to be weighed in air (W_a), and also weighed in water (W_w). Using Archimedes Principle: "a body immersed in water weighs less by the volume of water displaced", and a simple calculation, SG can be determined with substantial accuracy.

SG calculation: Weight of gem in air divided by the difference between the weight in air and the weight in water, or:

SG = W_a/ W_a-W_w

[Hydrostatic weighing set-up consisting of an electronic balance with a special hanging basket apparatus in which the gem can be suspended in water without putting weight on the scale.]

Again, an example. We have an unknown gem whose weight in air is 5.10 ct and whose weight in water = 3.20 ct. The difference in the air and water weights is 1.90 ct. Using the formula: SG = 5.10 ct/1.90 ct = 2.68. Looking in the tables at the back of the Hall book we quickly find several gem possibilities close to that SG: quartz (2.65), coral (2.68), aquamarine (2.69), and scapolite (2.70). More importantly, than what it might be, a SG of 2.65 rules out a large number of possibilities that it cannot be. The gemologist, like other scientists, progresses most often by weeding out wrong hypotheses (as opposed to proving right ones!).

Final Exam (just kidding!)

Scenario: We have obtained an unknown transparent green gem from a jeweler, the label has fallen off the box, and he/she would like us to tell them what it is. Since the gem was going to be used for jewelry, we can rule out the obscure and very soft collector gems, and limit our scope to relatively common jewelry gems that come in vivid, transparent green. This leaves emerald, chrome diopside, Tsavorite garnet and tourmaline as the prime suspects. We are just getting our gemology laboratory off the ground, so all we have is some reference books, a hydrostatic weighing set up, and a set of heavy liquids. First, we'll do our SG test hydrostatically, then with the heavy liquids.

We look up the SG ranges in our reference guides:

[Is it?: emerald (SG = 2.72 +.18/-.05); tourmaline (SG = 3.06 +.20/-.06); chrome diopside (SG = 3.29 +.11/-.07) or Tsavorite garnet (SG = 3.61 +.12/-.04)]

HYDROSTATIC TEST

STEP ONE: WEIGH GEM IN AIR

The weight in air is: 2.420 ct.

STEP TWO: ASSEMBLE HYDROSTATIC WEIGHING CHAMBER AND "TARE" BALANCE 

The beaker with water is actually suspended by an arm off to the side and does not put weight on the balance pan, the plastic ring which holds a little metal basket for the gem, does put weight on the balance, though. Once everything is set up, we "tare" the balance (resetting it to zero) so that it ignores the weight of the plastic ring and gem basket. Now we are ready to place the gem in the basket where it will be weighed underwater.

STEP THREE: WEIGH THE GEM IN WATER

The weight of the gem in water is: 1.615 ct. The difference between the weight in air and weight in water is: 2.420 ct - 1.615 ct = 0.805 ct

STEP FOUR: CALCULATE SG

SG = W_a/ W_a-W_w

SG = 2.420 ct /0.805 ct = 3.01

Can we eliminate any possibilites? Check the SG range of each of the four possibilities. (Assume we have made accurate measurements and our arithmetic is correct).

emerald (SG = 2.72 +.18/-.05); tourmaline (SG = 3.06 +.20/-.06); chrome diopside (SG = 3.29 +.11/-.07) or Tsavorite garnet (SG = 3.61 +.12/-.04)

The only gem whose SG range does not exclude 3.01 is: tourmaline! The others are either too high or too low to qualify. **Pretty cool.**

Testing by Heavy Liquids: Below are the results of the same test on the green gem, done with the set of heavy liquids:

3.32: gem floats rapidly

3.05 gem floats very slowly

2.67 gem sinks

2.62 gem sinks rapidly

2.52 gem sinks very rapidly

Based on these results, the conclusion we must draw is that the SG is below 3.05, and above 2.67 (but closer to 3.05). If this were the only available testing method, we would be able to eliminate the chrome diopside and the Tsavorite garnet, but we'd have to do some other tests to discriminate between emerald and tourmaline.

Most gemologists prefer to use the hydrostatic method, not only because of its greater precision, but also because the heavy liquids smell very bad, and have hazardous properties such that gloves and masks must be worn when using them.

HOMEMADE HEAVY LIQUID TEST FOR AMBER

[A saturated solution of salt water with amber and plastic immersed]

One fun, and safe, heavy liquid test that can be done at home uses a saturated saltwalter solution. (Make this by dissolving as much salt in room temperature distilled water as it will hold). The SG of this mini "Salt Lake" is about 1.13. Most types of natural amber will float in it (SG = 1.08) while nearly all the plastic materials used to make imitations of amber will sink as their SGs are higher than 1.13. Imitation amber is rampant in the gem marketplace (even in some of the better stores), so this is a handy trick to know. 

MISCELLANEOUS PHYSICAL PROPERTIES 

There is a very long list of esoteric physical properties which gemologists/physicists/crystallographers and others working in research labs can study and measure in gems and minerals. To finish up this section, though, I will mention just a few that have occasional usefulness for the ordinary gemologist or gem/jewelry lover, and that do not require high budget equipment.

1) Magnetism: Very few gems show any magnetic properties. One interesting exception is a certain type of synthetic diamond. In this case, a strong magnet can be a definitive way to separate these stones from natural diamonds.

Natural hematite is mildly to moderately attracted to a magnet, but an imitation version is so strongly magnetic that the difference is obvious.

2) Thermal Reaction: The response to high temperature in terms of appearance, and especially odor, can be a telling one in identifying some gems. Many organic gems such as horn, ivory, tortoise shell, and black coral smell like burning hair when touched with a "hot point" probe. Amber smells like turpentine, and jet like burning coal. Their common imitations may have odors, but not the right ones.

Although, technically destructive, this test can usually be done on a very small, inconspicuous spot. Resin, lacquer and wax coatings on gems can likewise be detected as they melt or char under the hot point. In this case, the reaction is best observed under magnification, with the hot probe not touching the surface, but just barely above it.

3) Thermal Conductivity: Gems differ quite dramatically in this property, which is basically a measure of the rate at which they conduct applied heat. For many years no savvy jeweler or pawn shop owner would be caught without a thermal conductivity tester (otherwise known as a diamond tester). By simply touching a small metal probe to the gem, it was instantly determined to be "diamond" or "not diamond". Pretty useful, huh? Well, it used to be....

A few years ago, two developments occured which have all but made these devices obsolete. 1) A new diamond simulant, called Moissanite whose thermal conductivity is close enough to diamond to pass the test, has come on the market, and 2) synthetic diamonds are now becoming a common enough to be concerned about. Man-made diamonds which have the same physical properties as the natural gems, would, of course, pass the test as diamond.

4) Electrical Conductivity: Very quickly upon the heels of the introduction of Moissanite, came the marketing of a new generation of testers which use a different tactic to separate Moissanite from diamond. Diamonds (with the vanishingly rare exception of natural blue ones) do not conduct electricity, but Moissanite does. So, out with the old and in with the new generation of diamond testers.

These machines have two systems, a thermal conductivity test, to first separate diamond and Moissanite from all other gems, then an electrical conductivity test to do the final separation should the thermal test indicate diamond. (Again, synthetic diamonds cannot be separated from natural ones with any basic physical tests).

[Mizar DiamonNite Dual Tester: Image courtesy of www.Mineralab.com]

Placing the probe on a gem initiates a thermal conductivity test to reveal CZ or other non-diamond simulants, then if the stone passes that, an electrical conductivity test follows to determine if it is diamond or Moissanite.

1) It is probably pink tourmaline, as tourmaline has no cleavage and Kunzite has two perfect cleavages. 

2) You should choose the blue sapphire. Sapphire has no cleavage and blue topaz has perfect cleavage in one direction.

3) Very thin or pointed areas on a cut gem, like the tips of a marquis cut, are areas of weakness; since apatite has cleavage, it would be much safer in a shape with smooth curves like a round or oval.

4) Seeing a conchoidal fracture pattern on the edge of the broken piece would indicate that is not turquoise (or coral) whose fractures are granular and uneven, respectively, but it could very well be glass.

5) The 6 mm Smithsonite is quite a bit heavier than the same sized calcite. The one carat calcite is noticeably larger than the same weight Smithsonite.

6) The 6 mm aragonite is a bit heavier than the same sized calcite. The one carat calcite is slightly larger than the same weight aragonite.

7) Even if gold and platinum were equally priced per ounce, the amount of platinum required for a given size and shape ring would weigh more (because it is denser) making the platinum ring more expensive.

You have now completed the web lecture for the third lesson! Go back the the course website to: 1) complete and submit the homework assignment on the text readings and assigned web essays 2) take the non-graded practice quiz on this web lecture 3) post a comment to the discussion board for this lesson, and 4) when it is available, complete the graded quiz based on this web lecture.When you're ready, proceed on to Lesson Four: Optical Properties of Gems