In the Name of Allahwho is the Master of life and death
Introduction to Gemology andPhysical Properties of Gemstone
Introduction to Gemology• Gemology (gemmology) is the science, art and profession of identifying and evaluating gemstones. It is considered a geoscienc and a branch of mineralogy.• Gemology is the study of gemstones. Some dictionaries define it as the “scientific study of gemstones,” but it is almost impossible to remove the scientific element. There are many categories of gemologists. For the jeweler it is a key element of their business. They need to be able to answer their customer’s questions and identify the gems brought into them.• Some jewelers are academically trained gemologists and are qualified to identify and evaluate gems.
Gemstone• A gem is a natural, mineral or organic substance, that has substantial beauty, rarity, and durability .
Natural Gemstone• Natural means that the material was not made, or assisted in its making, by human effort.
Mineral Gemstone• A mineral can be defined as a crystalline solid with a specific chemical formula, and a regular three dimensional arrangement of atoms. for example, opal and natural types of glass. [Faceted iolite, uncut emerald crystal]
Organic Gemstone• An organic gem is one that was made by living things, present or past. Examples include pearls, coral, jet, ivory, shell and amber. Such gems consist of the molecules formed by the organism, although these molecules may have been altered somewhat due to compression or 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 Gemstones• Properties of Gemstones: There are two sets of characteristics possessed by every gemstone, and by which they are studied, identified and evaluated: – 1) physical properties – 2) optical properties.
Physical Properties• Although there are a dozen or more physical properties which can be measured, in this course we will concentrate on just a few. In particular, our focus will be on those which are either visible directly, or measurable with minimal equipment, and those which are most important as indicators of a gems identity, and/or its suitability for particular uses:
Physical Properties• 1 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.
Physical Properties• 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: two, perfect (note extremely flat, smooth breaks), fluorite: four, perfect]
Physical Properties• 2 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 [Turquoise: granular, coral: uneven]
Physical Properties• 3 Hardness: The tendency to resist scratching in a gem is known as its hardnessMoh’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
Physical Properties[Talc, the softest on the Mohs scale, diamond, the hardest]
Physical Properties• 4 Toughness: The tendency to resist breaking and chipping is known as a gems toughness.• This property is controlled primarily by two factors: the readiness of a material to cleave in single crystal gems, and the presence or absence of certain structural characteristics in aggregate
Physical Properties• 5 Stability: Stability in a gem is a measure of its ability to resist changes due to exposure to light, heat and/or chemicals.• 6 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.
Physical Properties• 7 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 which must be kept constantly under opaque covers or else light exposure quickly renders its originally red color completely black.
Physical Properties• 8 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