gemology Physical Properties of Gem Stone.pptx

physical properties of gemstones

1. P HYSICAL CHARACTERISTICS OF GEM STONES F orm, Colour C leavage, F racture, H ardness, S pecific gravity and T enacity .

2. Some impoant definitions: Mineral: A mineral is a naturally occurring homogeneous solid with a definite chemical composition and definite crystal structure, usually formed by an inorganic process. - Definition of a mineral: Naturally occurring Inorganic solid Ordered internal molecular structure Definite chemical composition - Definition of a rock: A natural solid aggregate of minerals

3. Crystal : The term Crystal was first used by Greeks in 400 B.C. for transparent, crystallized rock-crystal (Quartz) that was found in the Alps and thought to be ice hardened due to intense cold. Crystal is a homogeneous solid bounded by plane surfaces (called faces) which are the external expression of a regular internal arrangement of constituent atoms or ions . A crystal must have ( i ) a regular geometric form and (ii) a characteristic internal atomic structure of which the later is an essential requirement while the former is developed under suitable physico -chemical conditions.

4. Some impoant definitions: G emstone : A gemstone is the naturally occurring crystalline form of a mineral, which is known for its beauty, valuable, rarity, and durable enough to be enjoyed for generations. G emology : Gemology is a branch of mineralogy dealing with natural and artificial gemstone materials. Some jewelers are academically trained gemologists and are qualified to identify and evaluate gems. Precious Gem: Diamond, Ruby, Sapphire, and Emerald (Opal and chrysoberyl are also considered as precious gems). Semiprecious Gem: Other varieties apart from the four are known as semiprecious gems. Diamond Ruby Saphire

5. Some important informations Measurement Unit: Gemstones are measured in terms of metric “carat ” which is equivalent to 200mg (1/5 g ) . Weighing measure in India : 1 Ratti = 0.91 carat 1 T ola = 64 rattis = 58.32 carets The term mangalin (1.75 carat) was also in use. The term ‘carat’ is often mistaken for a similar term used in the gold industry. But for gold, the carat/karat refers to purity (not the measurement). 24 carat denotes the purest form of gold. Specific Gravity is inversely proportional to their size . Ruby (SG 4.0) is smaller than the diamond (3.52). Gem Evaluation : Gems are evaluated in the concepts of 4-c (colour, clarity, cut and carat) and extend to 5-c (confidence).

6. Form/ Habit (Shape) Colour Luster Cleavage Fracture Hardness Streak Specific Gravity Diagnostic Properties Crystal System Chemical composition Physical Properties of Minerals Identifying minerals can be done by examining 11 characteristics that are unique to them. F orm, Colour C leavage, F racture, H ardness, S pecific gravity and T enacity .

7. Form and shape Every mineral has characteristic crystal form.

8. Form and shape Some other characteristic forms are: a. Columnar form – large crystals with equidimensional cross section (Tourmaline). b. Bladed form – flattened (Kyanite). c. Acicular – needle like (Natrolite). d. Fibrous – fibre (Asbestos, Gupsum). Columnar Bladed Acicular Fibrous

9. e. Radiated – diversing from common centre (Anthophyllite) F. Short – extended along a- and b- axis, appearing thin along c-axis.

10. Scalenohedron Calcite on Dolomite from the Elmwood Mine in Tennessee Lamellar crystal habit- iron roses Prismatic Calcite Bladed calcite Acicular Form Chrysotile fibrous crystal habit Botryoidal chalcedony Striated pyrite Radiating Malachit e Few othr examples

11. Colour Colour could be a diagonistic property of some mineral, but for others, presence of an impurity could affect the colour of the mineral e.g. colour of quartz

12. Cleavage Cleavage is the property of a mineral by which it shows tendency to break along one/more directions offering some closely smooth planes (called the cleavage planes). Cleavage occurs because the mineral has weaker bonds holding the atoms together in specific directions.

13. F racture Fracture - broken surface of a mineral in a direction other than that of cleavage . Even – Broken surface is smooth and flat, Uneven - Broken surface is irregular, Conchoidal – curved surface showing concentric lines)

14. Hardness Hardness is one of the most useful diagnostic properties of minerals. It measures a mineral's resistance to abrasion or scratching . To determine the hardness, a mineral of unknown hardness is rubbed against one of known hardness, and a numerical value can be obtained by using the Mohs scale.

15. S pecific gravity Specific Gravity: Mineralogists utilize a measure called specific gravity, which is related to density. It represents the ratio of a mineral's weight to the weight of an equal volume of water. Holding a mineral in your hand can estimate its specific gravity. If it feels similarly "heavy" to a mineral of similar size you have held before, then the sample's specific gravity is likely to be within the range of 2.5 to 3. Specific gravity can usually be qualitatively measured by the heft of a mineral, in other words those with high specific gravities usually feel heavier. Most common silicate minerals have a specific gravity between about 2.5 and 3.0. These would feel light compared to minerals with high specific gravities

16.   Mineral   Composition Spe c i f ic Gravity Graphite C 2.23 Quartz Si O 2 2.65 Feldspars ( K,N a )AlS i 3 O 8 2.6 - 2.75 Fluorite CaF 2 3.18 Topaz A l 2 S iO 4 ( F ,OH ) 2 3.53 Corundum A l 2 O 3 4.02 Barite BaS O 4 4.45 Pyrite FeS 2 5.02 Galena PbS 7.5 Cinnabar HgS 8.1 Copper Cu 8.9 Silver Ag 10.5 Quartz, a common rock-forming mineral, has a specific gravity of 2.65. Galena, a which is a metallic mineral and an ore of lead , has a specific gravity of 7.5.

17. Tenacity It is the r esistance offered by a mineral to breaking, crushing, banding or cutting . B rittle – break or powders as quartz; S ectile – plastically cut with a knife as gypsum; Ductile / F lexible - bends easily and does not return to its original shape. - - Chlorite, Talc; E lastic – assume original form after bending - Muscovite   Malleable - can be hammered into thin sheets.