Gem Stone and its Properties

In the Name of Allah In the Name of Allah who is the Master of who is the Master of

life and deathlife and death

Introduction to Introduction to Gemology and Gemology and

Physical Physical Properties of Properties of

GemstoneGemstone

Introduction to Introduction to GemologyGemology

• GemologyGemology ( (gemmologygemmology) is the science, art and ) is the science, art and profession of identifying and evaluating profession of identifying and evaluating gemstones. It is considered a geoscienc and a gemstones. It is considered a geoscienc and a branch of mineralogy.branch of mineralogy.

• Gemology is the study of gemstones. Some Gemology is the study of gemstones. Some dictionaries define it as the “scientific study of dictionaries define it as the “scientific study of gemstones,” but it is almost impossible to remove gemstones,” but it is almost impossible to remove the scientific element. the scientific element. There are many categories of gemologists. For the There are many categories of gemologists. For the jeweler it is a key element of their business. They jeweler it is a key element of their business. They need to be able to answer their customer’s need to be able to answer their customer’s questions and identify the gems brought into them.questions and identify the gems brought into them.

• Some jewelers are academically trained Some jewelers are academically trained gemologists and are qualified to identify and gemologists and are qualified to identify and evaluate gems.evaluate gems.

GemstoneGemstone

• A gem is a natural, mineral or A gem is a natural, mineral or organic substance, that has organic substance, that has substantial beauty, rarity, and substantial beauty, rarity, and durabilitydurability . .

• Natural means that the material Natural means that the material was not made, or assisted in its was not made, or assisted in its making, by human effort.making, by human effort.

Natural GemstoneNatural Gemstone

• A mineral can be A mineral can be defined as a defined as a crystalline solid crystalline solid with a specific with a specific chemical formula, chemical formula, and a regular three and a regular three dimensional dimensional arrangement of arrangement of atoms.atoms. for example, for example, opal and natural opal and natural types of glass.types of glass.

Mineral GemstoneMineral Gemstone

[Faceted iolite, uncut

emerald crystal]

Organic GemstoneOrganic Gemstone• An organic gem is one that was made by living An organic gem is one that was made by living

things, present or past. Examples include pearls, things, present or past. Examples include pearls, coral, jet, ivory, shell and amber. Such gems coral, jet, ivory, shell and amber. Such gems consist of the molecules formed by the consist of the molecules formed by the organism, although these molecules may have organism, although these molecules may have been altered somewhat due to compression or been altered somewhat due to compression or other geological or chemical forces.other geological or chemical forces.

[Organic gems: coral and freshwater cultured pearl earrings, faceted amber (enlargement showing fossilized insect within the gem]

Physical Properties of Physical Properties of GemstonesGemstones

• Properties of Gemstones:Properties of Gemstones: There There are two sets of characteristics are two sets of characteristics possessed by every gemstone, possessed by every gemstone, and by which they are studied, and by which they are studied, identified and evaluated: identified and evaluated:

– 1) physical properties1) physical properties– 2) optical properties.2) optical properties.

Physical PropertiesPhysical Properties

• Although there are a dozen or more Although there are a dozen or more physical properties which can be physical properties which can be measured, in this course we will measured, in this course we will concentrate on just a few. In concentrate on just a few. In particular, our focus will be on those particular, our focus will be on those which are either visible directly, or which are either visible directly, or measurable with minimal equipment, measurable with minimal equipment, and those which are most important as and those which are most important as indicators of a gem's identity, and/or indicators of a gem's identity, and/or its suitability for particular uses:its suitability for particular uses:

• 11 Cleavage:Cleavage: In the three dimensional In the three dimensional structure of certain crystals, atoms are structure of certain crystals, atoms are bound more tightly to each other in bound more tightly to each other in some directions and more loosely in some directions and more loosely in others. As a consequence, when strong others. As a consequence, when strong forces are applied, relatively clean forces are applied, relatively clean breaks may occur in these "weakest breaks may occur in these "weakest link" directions. These breaks, which link" directions. These breaks, which can sometimes be so smooth as to can sometimes be so smooth as to appear to have been polished, are appear to have been polished, are called cleavages.called cleavages.


• NotNot allall gemsgems showshow cleavage however, cleavage however, for example tourmalines, sapphires, for example tourmalines, sapphires, and garnets do not.and garnets do not.

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



• 22 Fracture:Fracture: Whereas cleavages Whereas cleavages occur only in some occur only in some gems, and within gems, and within those, only in those, only in certain directions, certain directions, fractures can, and fractures can, and do, occur in all do, occur in all gems, and in any gems, and in any direction. A direction. A fracture is a break fracture is a break which is which is notnot along along a cleavage planea cleavage plane

[Turquoise: granular, coral: uneven]

Physical PropertiesPhysical Properties• 33 Hardness:Hardness: The tendency to resist scratching in a gem is The tendency to resist scratching in a gem is

known as its hardnessknown as its hardness

Moh’s Scale of Hardness:Moh’s Scale of Hardness: Hardness

Mineral Absolute Hardness

1 Talc (Mg3Si4O10(OH)2) 1

2 Gypsum (CaSO4·2H2O) 2

3 Calcite (CaCO3) 9

4 Fluorite (CaF2) 21

5 Apatite (Ca5(PO4)3(OH-,Cl-,F-) 48

6 Orthoclase Feldspar (KAlSi3O8) 72

7 Quartz (SiO2) 100

8 Topaz (Al2SiO4(OH-,F-)2) 200

9 Corundum (Al2O3) 400

10 Diamond (C) 1500


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


• 44 Toughness:Toughness: The tendency to The tendency to resist breaking and chipping is resist breaking and chipping is known as a gem's toughness.known as a gem's toughness.

• This property is controlled primarily This property is controlled primarily by two factors: the readiness of a by two factors: the readiness of a material to cleave in single crystal material to cleave in single crystal gems, and the presence or absence gems, and the presence or absence of certain structural characteristics of certain structural characteristics in aggregatein aggregate


• 55 StabilityStability: Stability in a gem is a : Stability in a gem is a measure of its ability to resist changes measure of its ability to resist changes due to exposure to light, heat and/or due to exposure to light, heat and/or chemicals.chemicals.

• 66 DehydrationDehydration: Heat is a factor that : Heat is a factor that can create problems with certain gems. In can create problems with certain gems. In some cases, the mineral comprising the some cases, the mineral comprising the gem is "hydrated", that is, it contains gem is "hydrated", that is, it contains water molecules which adhere chemically water molecules which adhere chemically with varying degrees of tenacity.with varying degrees of tenacity.

• 77 Light:Light: Some gems can fade Some gems can fade or change color when exposed to or change color when exposed to light. An extreme example of this light. An extreme example of this phenomenon is seen in the rare phenomenon is seen in the rare mineral pyrargyrite which must mineral pyrargyrite which must be kept constantly under opaque be kept constantly under opaque covers or else light exposure covers or else light exposure quickly renders its originally red quickly renders its originally red color completely black.color completely black.


• 88 Specific Gravity: Specific Gravity: Specific gravity, also Specific gravity, also known as relative density, differs widely known as relative density, differs widely among gemstones, and is one of their most among gemstones, and is one of their most important physical characteristics from the important physical characteristics from the viewpoint of gem identification. Specific viewpoint of gem identification. Specific gravity (SG) is the ratio of the weight of one gravity (SG) is the ratio of the weight of one unit volume of the gem to the weight of the unit volume of the gem to the weight of the same unit of water. For example, to say same unit of water. For example, to say sapphire (corundum) has SG = 4.0, means sapphire (corundum) has SG = 4.0, means precisely that a cubic inch of sapphire precisely that a cubic inch of sapphire weighs four times as much as a cubic inch of weighs four times as much as a cubic inch of waterwater


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ReferencesReferences

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Gem Stone and its Properties

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