AGY 509AGY 509GEMSTONES TECHNOLOGYGEMSTONES TECHNOLOGY

(2 UNITS)(2 UNITS)

LECTURER: LECTURER: A.Y.B. ANIFOWOSE, A.Y.B. ANIFOWOSE, PhDPhDDepartment of Applied Geology,Department of Applied Geology,

Federal University of Technology, Akure.Federal University of Technology, Akure.Corresponding Email Address: aybanifowose@futa.edu.ng; Corresponding Email Address: aybanifowose@futa.edu.ng;

yanifowose@yahoo.comyanifowose@yahoo.com

SynopsisSynopsis•Crystal form and habit, TwinningCrystal form and habit, Twinning•Isomorphism and isomorphous replacement.Isomorphism and isomorphous replacement.•Determination of specific gravity and Determination of specific gravity and refractive index of crystals.refractive index of crystals.•Optical properties. Colours; causes and Optical properties. Colours; causes and artificial alteration, diffraction of light. artificial alteration, diffraction of light. Spectroscopy and absorption spectra.Spectroscopy and absorption spectra.•Luminescent and electrical properties. Luminescent and electrical properties. Application of X-rays and ultraviolet light to Application of X-rays and ultraviolet light to gem testing.gem testing.•Inclusions and other internal features. Inclusions and other internal features.

•Description of gem minerals.Description of gem minerals.•Natural gemstones; colour, chemical composition, Natural gemstones; colour, chemical composition, crystal system cleavage, hardness, specific gravity, crystal system cleavage, hardness, specific gravity, refractive index, pleochroism, lustre, mode of refractive index, pleochroism, lustre, mode of occurrence.occurrence.•Sources of different gem varieties and methods of Sources of different gem varieties and methods of identification.identification.•Synthetic and Imitation stones: Synthetic, Synthetic and Imitation stones: Synthetic, composite and paste stones; methods of composite and paste stones; methods of manufacture and identification.manufacture and identification.•Organic Products: Native and cultured pearl; their Organic Products: Native and cultured pearl; their formation, structure and occurrence. Imitation formation, structure and occurrence. Imitation pearl. Methods of identification.pearl. Methods of identification.•Occurrence and identification of Amber; coral, Occurrence and identification of Amber; coral, ivory, tortoise's shell etc.ivory, tortoise's shell etc.•Fashioning of gemstone: Outline of methods used Fashioning of gemstone: Outline of methods used by diamond cutters and lapidaries. by diamond cutters and lapidaries.

Synopsis contd.Synopsis contd.

List of BooksList of BooksWebster, R. Gems, their sources, descriptions and Webster, R. Gems, their sources, descriptions and Identification. 5Identification. 5thth Edition.Edition.Read, H.H. Rutley’s Elements of Mineralogy. 27Read, H.H. Rutley’s Elements of Mineralogy. 27thth Edition, Edition, Thomas Thomas Murby & Co., London.Murby & Co., London.Blackburn, W.H. and Dennen, W.H. Principles of Mineralogy. Blackburn, W.H. and Dennen, W.H. Principles of Mineralogy. Universal Universal Book Stall, India.Book Stall, India.Read, H.H. and Watson, J. Introduction to Geology. Vol. 1: Read, H.H. and Watson, J. Introduction to Geology. Vol. 1: Principles. Principles. Macmillan, London.Macmillan, London.Illustrated Guide to Jewelry Appraising. 3Illustrated Guide to Jewelry Appraising. 3rdrd Edition. Edition.Matlins, A. Gem Identification made Easy. 4Matlins, A. Gem Identification made Easy. 4thth Edition. Edition.www.allaboutgemstones.comwww.allaboutgemstones.com

FORMATION OF A FORMATION OF A CRYSTALCRYSTAL

Crystals are bodies which are bounded by plane surfaces usually flat, and arranged on a definite plan (i.e. the lattice or unit cell) as an expression of the internal arrangement of atoms.The unit cell is the smallest divisible unit of a given mineral with symmetrical characteristics that are unique to its crystalline structure. A structure's unit-cell is a spatial arrangement of atoms (motifs) which are "tiled" in a three-dimensional space to form the crystal.These crystals are formed by the process known as crystallization which is the solidification of minerals from a magma or post-magmatic environments.Natural substances are of TWO types: crystalline (those with internal orderliness in atom arrangement); amorphous (those without orderliness).

ASSIGNMENASSIGNMENTT

• WRITE ABOUT THE FOLLOWING:WRITE ABOUT THE FOLLOWING:• Ionic/polar bond (between ions)Ionic/polar bond (between ions)• Covalent bonding (electron sharing)Covalent bonding (electron sharing)• Metallic bonding (cohesion of metals, i.e. cubic Metallic bonding (cohesion of metals, i.e. cubic

close packing e.g. Cu; body-centered cubic close packing e.g. Cu; body-centered cubic packing e.g. Fe; hexagonal close packing e.g. packing e.g. Fe; hexagonal close packing e.g. Mg)Mg)

• Residual bonding (weak forces present in all Residual bonding (weak forces present in all crystals)crystals)

• Polymorphism (comparison of diamond and Polymorphism (comparison of diamond and graphite).graphite).

CRYSTAL CHEMISTRY

In ionic crystals, cations are surrounded by certain number of anions, and at a distance according to the atomic radii of those atoms. The number of cations surrounding an anion (or vice versa) is called the Coordination Number, which is determined by the ratio of the radii of the two ions.Crystal formation depends on two main factors: electrical neutrality and ionic radii, the former being responsible for attraction between different ions and repulsion between like ions. The ionic radii guarantee optimum distance requirement for stable compounds’ formation.

Crystal Crystal MorphologyMorphology

Crystals are bounded by a number of surfaces which are usually perfectly flat but may be curved, e.g. some specimens of siderite and diamond.These surfaces are called faces. Faces with same properties are called like faces, while those with different properties are called unlike faces.A crystal with like faces is called a simple form, while a crystal of two or more groups of faces is called a combination.Other characteristics are: edge, solid angle, and interfacial angle.

Crystal SystemCrystal SystemAll gemstones are crystalline structures made from a mixture of different elemental compounds, and the shape of a crystal is based on the atomic structure of these elemental building blocks.A gem's crystal structure will determine its symmetry, optical properties, cleavage planes, and overall geometric shape. The recipe, or mixture of these compounds becomes the blueprint for how the crystal will grow. A crystal's growth pattern is referred to as its "Crystal Habit."The crystal system is a grouping of crystal structures that are categorized according to the axial system used to describe their "lattice". A crystal's lattice is a three dimensional network of atoms that are arranged in a symmetrical pattern.

Crystal System Crystal System contd.contd.

•Also known as isometric system.•It is characterized by its total symmetry. •Three crystallographic axes that are all perpendicular to each other, and equal in length.•It has one lattice point on each of the cube's four corners.

Cubic System

Crystal System Crystal System contd.contd.

•It has four crystallographic axes consisting of three equal horizontal or equatorial (a, b, and d) axes at 120o, and one vertical (c) axis that is perpendicular to the other three.•The (c) axis can be shorter, or longer than the horizontal axes.

Hexagonal System

The crystal is a simple cubic shape that is stretched along its (c) axis to form a rectangular prism. The tetragonal crystal will have a square base and top, but a height which is taller. By continuing to stretch the "body-centered" cubic, one more Bravais lattice of the tetragonal system is constructed.

Crystal System Crystal System contd.contd.

Tetragonal System

Crystal System Crystal System contd.contd.

Rhombohedral System•It is also called Trigonal system, with a three-dimensional shape that is similar to a cube, but it has been skewed or inclined to one side making it oblique.•Its form is considered prismatic because all six crystal faces are parallel to each other.•Any faces that are not squared at right angels are called "rhombi.“•A rhombohedral crystal has six faces, 12 edges, and 8 vertices.•If all of the non-obtuse internal angles of the faces are equal (flat sample, below), it can be called a trigonal-trapezohedron.

Crystal System Crystal System contd.contd.Orthorhombic System

•It is also called Rhombic system.•The crystal system has three mutually perpendicular axes, all with different, or unequal lengths.

Crystal System Crystal System contd.contd.

Monoclinic System•The crystals have three unequal axes: The (a) and (c) crystallographic axes are inclined toward each other at an oblique angle, and the (b) axis is perpendicular to a and c.•The (b) crystallographic axis is called the "ortho" axis.

Crystal System Crystal System contd.contd.

Triclinic System

•The crystals have three unequal crystallographic axes, all of which intersect at oblique angles. •Triclinic crystals have a 1-fold symmetry axis with virtually no discernible symmetry, and no mirrored or prismatic planes.

TwinningTwinning

•Some substances commonly crystallise as composite crystals of a sort known as twinned crystals.•Twinning occurs when two or more crystals intergrow in a symmetrical arrangement.•Well-known examples are fluorite, calcite, copper and diamond.•A twinned consists of two or more individual single crystals joined together in some definite mutual orientation, such that the lattice of one individual is related to that of the other individual(s) in the composite crystal by some simple symmetry operation.

Twinning Twinning contd.contd.

•Therefore it looks as if they have been divided along a plane and one half orientated 180o, manifesting in opposite sides being brought to the same end or distance away from the plane.•One part of the twin looks like a mirror image of the other.•The plane of reflection is called the twin plane, while the axis about which the crystals can be rotated is known as the twin axis which is usually at right angles to the twin plane.

•Where the two components of a twinned crystal are joined in a plane, the crystal is called a contact twin and the plane of mutual contact, which is a lattice plane, is known as the composition plane. In general, it is true to say that if the twin axis is a zone axis, then the composition plane is parallel to the twin axis.•In reflexion twins, the composition plane is parallel to the twin plane.

Twinning Twinning contd.contd.

Twinning Twinning contd.contd.Types of Twin Crystals

1)Growth twins, Deformation (or glide) twins, transformation (or inversion) twins.2)(a) Simple twin: two halves symmetrical about the twin plane, e.g. calcite and rutile.

(b) Repeated twin: this is produced by repetition of twinning, e.g. albite twinning with parallel twin planes.

(c) Penetration twin: this type has two crystals which cannot be actually separated into two, e.g. fluorite (CaF2)

Twinning Twinning contd.contd.

Formation of Twinned CrystalsAs a crystal grows from its initial nucleus, some accident of growth may cause it to twin; such accidents being for a variety of reasons very much more probable in some structures than in others.Twinning may alternatively provide a means of relieving the strain induced by some applied stress.Twinning may also be produced as the result of polymorphic transformations when a structure of higher symmetry is converted to a structure of lower symmetry on cooling.

Determination of specific gravity

•The specific gravity (or relative density) of a substance is its weight compared with that of an equal volume of pure water at a temperature of 4oC. The density of a substance is its weight per unit volume. The temperature of 4oC is chosen because this is the critical point at which water is most dense.

•For exact determinations various weighing devices are used.•Rough approximations of the specific gravity of lighter gemstones can be made by means of a series of liquids of known specific gravity.

Determination of specific gravity

Determination of specific gravity contd.

Methods of measurement•Hydrostatic method: It is the best means for getting accurate results but requires the use of sesitive balance.•Heavy liquid method: If the gemstone will float in a liquid having a specific gravity of 4, and sink in a liquid with a specific gravity of 3, the specific gravity of the gemstone must lie between these limits and be approximately 3.5. Liquids that are usually used are: Bromoform, CHBr3 (for SG ranging between 1.0 and 4.15); di-iodomethane (also called methylene iodide, CH2I2); and an aqueous solution of thallium salts called Clerici solution.

Methods of measurement contd.An alternative to bromoform is acetylene tetrabromide (CHBr2).Sodium polytungstate (3Na2WO4.9WO4.H2O) has been proposed as a safer alternative to some of the organic heavy liquids. It has SG of about 3.10, although it can be diluted with water to 3.05 to test for tourmaline, or to 2.65 for quartz.

Determination of specific gravity contd.

Refractive Index of crystals

When light passes from one density or type of material (air) into another (the gemstone), it is bent or "refracted." The amount, or degree, that the beam of light is bent will be determined by the density difference between the air and the gem. The measurement used to quantify the amount that a light beam is bent in a given material is known as its refractive index (R.I.). The gemologist uses an instrument called a refractometer to measure this characteristic property.

Refractive Index of crystals contd.

The incident angle is the angle of the approaching light as it intersects with the stone's exterior surface.The refractive angle is the altered angle of the light as it passes through the stone's interior.The Refractive Index is the ratio of difference between these two angles.Each material has its own unique density and Refractive Index.

VISUAL LOOK OF VISUAL LOOK OF GEMSTONESGEMSTONES

The value of gemstones generally depends on four elements:1.the beauty of the stone itself;2.its rarity;3.its hardness and toughness;4.the skill with which it has been cut and polished.

VISUAL LOOK OF GEMSTONESThe visual appeal and characteristics of a gemstone are determined by:

1.Brilliance (sparkle)2.Color3.Fire (light dispersion)4.Luster (surface reflectiveness).

A stone's brilliance, fire, and to a lesser extent luster, are influenced by the type of cut used.The color of a gemstone is due to one or more of several key factors. A mineral can have its own inherent color due to its basic chemical makeup (idiochromatic), or it can start out as a colorless material and gain its characteristic color from trace impurities (allochromatic). A mineral can also display multi-colored characteristics from the scattering or dispersion of light (pseudochromatic) that is reflected from its surface, while having little or no inherent color.

• The actual color of a gemstone (not including any optical effects The actual color of a gemstone (not including any optical effects caused by reflection or refraction) is caused by reflection or refraction) is due to the absorptive and due to the absorptive and reflective properties of any trace elements and impurities within the reflective properties of any trace elements and impurities within the crystal.crystal.

• Basic elements such as chromium, copper, iron, manganese, and Basic elements such as chromium, copper, iron, manganese, and vanadium absorb and/or reflect different wavelengths of the color vanadium absorb and/or reflect different wavelengths of the color spectrum playing an integral part in affecting the color of the mineral.spectrum playing an integral part in affecting the color of the mineral.

• Color in gemstones can be caused by chromophores (trace Color in gemstones can be caused by chromophores (trace impurities), crystallographic (electronic) distortions in the stone's impurities), crystallographic (electronic) distortions in the stone's matrix, or a combination of both.matrix, or a combination of both.

Gemstone Color Caused by Absorption or Reflection

Perceived Color & Metamerism

• Materials do not actually contain colorMaterials do not actually contain color, but they do have a , but they do have a perceived color when placed under different light sources, perceived color when placed under different light sources, which is a result of the light that is returned or extinguished.which is a result of the light that is returned or extinguished.

• The difference or similarity of a material's color under different The difference or similarity of a material's color under different light sources is referred to as "metamerismlight sources is referred to as "metamerism." ."

• In colorimetry, In colorimetry, metamerism refers to the matching of the metamerism refers to the matching of the "apparent" color of materials that have different spectral power "apparent" color of materials that have different spectral power distributions which describe the proportion of total light that is distributions which describe the proportion of total light that is emitted, transmitted, or reflected by a material at every visible emitted, transmitted, or reflected by a material at every visible wavelength.wavelength.

• The human eye contains only three color receptors called The human eye contains only three color receptors called “cone “cone cells”cells”. Therefore all visible colors are reduced to three sensory . Therefore all visible colors are reduced to three sensory quantities, called "tristimulus values." Metamerism occurs quantities, called "tristimulus values." Metamerism occurs because each type of cone responds to the cumulative energy because each type of cone responds to the cumulative energy from a broad range of wavelengths, so that different from a broad range of wavelengths, so that different combinations of light across all wavelengths can produce an combinations of light across all wavelengths can produce an equivalent receptor response and the same tristimulus values equivalent receptor response and the same tristimulus values or color sensation.or color sensation.

OPTICAL PROPERTIES OF OPTICAL PROPERTIES OF GEMSTONESGEMSTONES

• MMost important optical properties areost important optical properties are:: degree of refraction and degree of refraction and color.color. Other properties include Other properties include firefire (the display of prismatic (the display of prismatic colors), colors), dichroismdichroism (the ability of some gemstones to present two (the ability of some gemstones to present two different colors when viewed in different directions), and different colors when viewed in different directions), and transparencytransparency..

• Diamond is highly prized because of its fire and brilliancy, ruby Diamond is highly prized because of its fire and brilliancy, ruby and emerald because of the intensity and beauty of their colors, and emerald because of the intensity and beauty of their colors, and star sapphire and star ruby because of the star effect, known and star sapphire and star ruby because of the star effect, known as asterism, as well as for their color. as asterism, as well as for their color.

• In certain gemstones, notably opals, brilliant areas of color can be In certain gemstones, notably opals, brilliant areas of color can be seen within the stone; these areas change in hue and size as the seen within the stone; these areas change in hue and size as the stone is moved. This phenomenon, known as stone is moved. This phenomenon, known as play of colorplay of color, differs , differs from fire and from fire and is caused by interference and reflection of the light is caused by interference and reflection of the light by tiny irregularities and cracks inside the stone.by tiny irregularities and cracks inside the stone.

Opals also exhibit milky or smoky reflections from within the gem. Gems that are fibrous in structure show irregular interior reflections similar to those seen on watered or moiré silk.This optical property, which is called chatoyancy, is exhibited by several gems, notably the tiger-eye and cat’s-eye.The appearance of a gem as seen by reflected light is another optical property of gemstones and is called luster. The luster of gems is characterized by the terms metallic, adamantine (like the luster of the diamond), vitreous (like the luster of glass), resinous, greasy, silky, pearly, or dull. Luster is particularly important in the identification of gemstones in their uncut state.

OPTICAL PROPERTIES OF OPTICAL PROPERTIES OF GEMSTONES GEMSTONES contd.contd.

OPTICAL PROPERTIES OF GEMSTONES OPTICAL PROPERTIES OF GEMSTONES contd.contd.

• The beauty of gems depends to a large The beauty of gems depends to a large extent on their optical properties. The extent on their optical properties. The most important optical properties are the most important optical properties are the degree of refraction and color. Other degree of refraction and color. Other properties include fire (the display of properties include fire (the display of prismatic colors), dichroism (the ability of prismatic colors), dichroism (the ability of some gemstones to present two different some gemstones to present two different colors when viewed in different directions), colors when viewed in different directions), and transparency. and transparency.

• In certain gemstones, notably opals, In certain gemstones, notably opals, brilliant areas of color can be seen brilliant areas of color can be seen within the stone; these areas change in within the stone; these areas change in hue and size as the stone is moved. hue and size as the stone is moved. This phenomenon, known as play of This phenomenon, known as play of color, differs from fire and is caused by color, differs from fire and is caused by interference and reflection of the light interference and reflection of the light by tiny irregularities and cracks inside by tiny irregularities and cracks inside the stone.the stone.

OPTICAL PROPERTIES OF GEMSTONES OPTICAL PROPERTIES OF GEMSTONES contd.contd.

IDENTIFICATION OF GEMSIDENTIFICATION OF GEMS

• A gem cannot always be identified by sight alone.A gem cannot always be identified by sight alone.• The gemologist uses an instrument called a The gemologist uses an instrument called a

refractometer to measure the characteristic refractometer to measure the characteristic property of the stone, known as refractive index, property of the stone, known as refractive index, which is its relative ability to refract light.which is its relative ability to refract light.

• Emeralds, rubies, sapphires, amethysts, and Emeralds, rubies, sapphires, amethysts, and synthetic rubies and sapphires are all doubly synthetic rubies and sapphires are all doubly refracting, whereas diamonds, spinels, synthetic refracting, whereas diamonds, spinels, synthetic spinels, garnets, and glass are singly refracting.spinels, garnets, and glass are singly refracting.

GEMSTONE REFRACTION INDEXColor, Dispersion, and Reflectiveness:The visual appeal and characteristics of a gemstone are determined by several factors that include: brilliance (sparkle), color, fire (light dispersion), and luster (surface reflectiveness).The faceting of a gemstone will affect how light will behave as it passes through the outside surface into the interior of the gem.As a light beam passes through a gem it is bent or "refracted" before it exits the crystal. The light beam is also broken into its component parts (dispersion) causing the effect known as "fire.“As the stone is moved, the refraction and reflection points of the facets change, showing the stone's scintillation or "play of color.“

Artificial Gem: Artificial Gem: The term “artificial gem” The term “artificial gem” is used tois used to describe either an imitation describe either an imitation of a natural gemstone or a synthetic of a natural gemstone or a synthetic gem that is chemically identical to gem that is chemically identical to naturally occurring gems.naturally occurring gems. There are There are two types: two types: Imitation and Synthetic Imitation and Synthetic types.types.

TYPES OF GEMS

Imitation GemsSuch a gem may be made of flint glass, often silvered on the back to increase the brilliance.Since World War II, colored plastics have replaced glass, especially in costume jewelry. Plastics are cheaper, more easily molded, and lighter in weight. During the 19th century, artificial pearls were made by blowing hollow beads of glass and pouring into them a mixture of liquid ammonia and the white matter from the scales of fish such as the bleak, roach, or dace.

The most successful imitation of a diamond is strontium titanate, made by a flame-fusion technique. Its RI is almost identical to that of diamond, and it has a higher dispersion. Thus, it has the brilliance and greater fire than diamond. It scratches easily, however. A harder material simulating the diamond is rutile, or titanium oxide.

Imitation Gems contd.

• These are These are manufactured materials that duplicate a manufactured materials that duplicate a natural gemstone chemically, physically, and optically. natural gemstone chemically, physically, and optically. Synthetic gems can be distinguished under a microscope Synthetic gems can be distinguished under a microscope because they are more perfect than natural gemstones because they are more perfect than natural gemstones and contain no irregularities.and contain no irregularities.

• The diamonds thus produced are suitable only for The diamonds thus produced are suitable only for industrial use. In the late 1960s a method was developed industrial use. In the late 1960s a method was developed for “growing” diamonds by heating a diamond particle to for “growing” diamonds by heating a diamond particle to a high temperature and subjecting it to methane gas.a high temperature and subjecting it to methane gas.

• The gas decomposes into carbon atoms, which adhere to The gas decomposes into carbon atoms, which adhere to the diamond crystal. The crystal structure of the the diamond crystal. The crystal structure of the enlarged diamond is identical to that of a natural enlarged diamond is identical to that of a natural diamond.diamond.

Synthetic Gems

]

•Sapphires are made in an apparatus resembling an oxyhydrogen torch. The flame is directed into a fireclay support inside an insulated chamber. The oxygen gas carries finely powdered pure aluminum oxide into the flame, and the powder fuses into droplets, forming a cylindrical boule, or matrix, on the support.•The size of the resulting sapphire is controlled by varying the gas flow, temperature, and amount of powder.•Colors other than red are produced by adding different metallic oxides. Stars can be added to synthetic rubies or sapphires by adding an excess of titanium oxide to the aluminum oxide powder and heating to temperatures greater than 1000°C.

Synthetic Gems contd.

• In its widest sense the term In its widest sense the term jewelryjewelry encompasses encompasses objects made of many kinds of organic and objects made of many kinds of organic and inorganic materials such as hair, feathers, inorganic materials such as hair, feathers, leather, scales, bones, shells, wood, ceramics, leather, scales, bones, shells, wood, ceramics, metals, and minerals.metals, and minerals.

• Jewelry, ornaments of precious metal, sometimes Jewelry, ornaments of precious metal, sometimes set with gems, worn since ancient times by set with gems, worn since ancient times by people of all cultures for personal adornment, as people of all cultures for personal adornment, as badges of social or official rank, and as emblems badges of social or official rank, and as emblems of religious, social, or political affiliation. of religious, social, or political affiliation.

JEWELRYJEWELRY

The term properly refers to mounted precious or semiprecious stones and to objects made of valuable or attractive metals such as gold, silver, platinum, copper, and brass.

JEWELRYJEWELRY contd.contd.

Jewelry has been wornJewelry has been worn:: on the head in the form of on the head in the form of crowns, diadems, tiaras, , diadems, tiaras,

aigrettes, hairpins, hat ornaments, aigrettes, hairpins, hat ornaments, earrings, nose rings, , nose rings, earplugs, and lip rings;earplugs, and lip rings;

on the neck in the form of collars, necklaces, and on the neck in the form of collars, necklaces, and pendants;pendants;

on the breast in the form of pectorals, brooches, clasps, on the breast in the form of pectorals, brooches, clasps, and buttons; and buttons;

on the limbs in the form of on the limbs in the form of rings, , bracelets, armlets, and , armlets, and anklets;anklets;

and at the waist in the form of belts and girdles, with and at the waist in the form of belts and girdles, with pendants such as chatelaines, scent cases, and rosaries.pendants such as chatelaines, scent cases, and rosaries.

JEWELRYJEWELRY contd.contd.

GEMSTONES CUTTINGGEMSTONES CUTTING &&

EQUIPMENT REQUIREDEQUIPMENT REQUIRED• Cutting a raw stone into a faceted and polished Cutting a raw stone into a faceted and polished

gemstone is a multi-step process.gemstone is a multi-step process.• Each step is critical to the final outcome.Each step is critical to the final outcome.• The steps are:The steps are:

•MarkingMarking•CleavingCleaving•SawingSawing•Girdling (Bruting)Girdling (Bruting)•Faceting (Cross Working)Faceting (Cross Working)•Brillianteering.Brillianteering.

Marking: A rough stone is marked prior to cleaving or sawing to determine the direction of the grain or "cleavage", eliminate waste, and bypass inclusions or imperfections. The natural shape of the rough stone will also be a major factor in deciding how to cut the stone.

GEMSTONES CUTTING contd.

GEMSTONES CUTTING contd.

Cleaving: This refers to splitting a stone along its grain by striking it. A rough stone is cleaved if there are conspicuous defects and/or inclusions which would prevent it from being made into a single gemstone.

GEMSTONES CUTTING contd.Sawing: The rough stone is

cut to a shape that approximates the shape of the finished cut stone but without the facets.

GEMSTONES CUTTING contd.Girdling: The rough stone is placed in a chuck on a

lathe. While the rough stone rotates on the lathe, a second diamond mounted on a dop is pressed against it, rounding the rough diamond into a conical shape. This step is also referred to as rounding or bruting.

AutoBruter and display on a computer screen

GEMSTONES CUTTING contd.Faceting: The cutting and polishing of each

facet is accomplished by attaching the stone to a dop stick and pressing it against a revolving lap. During this faceting stage the angles of each facet must be cut in order to maintain symmetry and produce maximum brilliance.

Brillianteering: If the primary faceting or "cross-working" is done by a separate craftsman, the final 40 facets of a round brilliant cut diamond's 58 facets will be cut by a Brillianteer.

GEMSTONES CUTTING contd.

Dia-Analyser uses a camera to take photographs of the finished diamond while it is being rotated.

OTHER EQUIPMENT USED IN LAPIDARY

The Poly-Metric Scintillator 88 Digital machine (left) and the Facetron (right) are semi-automated faceting machines. They are water-cooled machines designed to make cuts at precise angles by plotting out to depth and degree of a given facet. The rough gemstone is held by a chuck called a "Dop stick" or "Dop chuck" and ground against a grinding plate called a "lap".

Rough gemstone attached to a dop stick (Center bottom).Lapidary gemstone saw (slab saw) and grinder (Right).

OTHER EQUIPMENT USED IN LAPIDARY contd.

Cabochon Cabochon Grinder/TumblerGrinder/Tumbler

OTHER EQUIPMENT USED IN LAPIDARY contd.

In order to understand why a gem is faceted or cut en cabochon, it is important to understand how light will behave once it passes into a gemstone or is reflected off its surface.

Reflection and Refraction in Faceting

The "critical angle" calculation is used to determine how facets should be placed in relation to each other in order to control the path of light within a gemstone.

Optical effects that are the result of light being refracted are

•Spectral dispersion•Double refraction (and/or reflected)•Light scattering from the gemstone.

1. Pseudochromatic Coloration: Caused from optics effects created by spectral dispersion:

•Fire: Diamond, Zircon 2. Double Refraction: Caused by incident light being split into two separate rays:

•Birefringence: Calcite, Moissanite, Zircon 3. Pseudochromatic Coloration: Caused from optics effects created by light scattering:

•Chatoyancy: Alexandrite, Tiger's Eye •Asterism: Garnet, Star Ruby, Star Sapphire •Luster: Pearl, Talc, Gypsum •Aventurescence: Sunstone, Aventurine Quartz •Adularescence: Moonstone (blue), Opal (milky white)

CRYSTAL HABITS OF SOME GEMSTONES

Crystal Habit: Habit Description: Mineral:

Acicular Needle-like, slender

and/or tapered

Rutile (Quartz)

Anhedral Poorly formed and

distorted

Olivine

Banded narrow bands of

differing colors

Onyx

Bladed Slender, flattened and

blade-like

Tanzanite

Columnar Long, slender prisms

and parallel growth

Calcite

Dendritic Tree-like multi-

directional branching

from central point

Opal

Dodecahedral 12-sided

Dodecahedron

Garnet

Drusy (Encrustation) Aggregate of minute

crystals coating a

surface

Drusy (Quartz)

Enantiomorphic Mirror-image

(left/right) habit and

optical characteristics

Citrine, Amethyst

Equant (Stubby) Squashed, pinnacoids

dominant over prisms

Zircon

Euhedral Well-formed and

Undistorted

Spinel

Foliated Easily separated into

plates

Mica

Geode Rock cavities with

internal crystals &

encrustation

Amethyst, Quartz

Granular Mass Anhedral crystals in

lumpy mass with no

crystal form

Peridot

Mamillary Intersecting large

rounded contours

Malachite

Massive Shapeless with no

distinct external

crystal shape

Lapis Lazuli

CRYSTAL HABITS OF SOME GEMSTONES contd.

Octahedral Eight-sided octahedron -

two pyramids, base to

base

Diamond

Pinacoid Terminated by multiple

pyramidal faces

Emerald, Beryl

Prismatic Elongate and Prism-Like,

all faces are parallel to c-

axis

Tourmaline, Topaz

Pseudomorphous Occurring in the shape of

another mineral

Tiger's Eye

Striated Surface growth lines

parallel or perpendicular

to c-axis

Alexandrite

Tabular (Lamellar) Flat, tablet-shaped,

prominent pinnacoid

Ruby

CRYSTAL HABITS OF SOME GEMSTONES contd.

1. Acicular Crystal Habit

Three variations of Acicular Rutilated Quartz

2. Bladed Crystal Habit

Bladed Tanzanite (center) and Kyanite (right)

3. Columnar Crystal Habit

Columnar Quartz (left), Smoky Quartz (center), and Basalt Rock (Yellowstone, Wyoming) Formations (right)

4. Dendritic Crystal Habit

Dendritic Agate (left), Opal (center), and Agate (right)

5. Dodecahedral Crystal Habit

Dodecahedral Garnet (center), Diamond Rough (right)

6. Drusy Crystal Habit

Three variations of Drusy Quartz

7. Enantiomorphic Crystal Habit

Enantiomorphic ("Japan Law" or "Left-Right" Twins) Quartz (center), Amethyst (right)

8. Equant Crystal Habit

Equant Zircon Roughs (center, and right)

9. Euhedral Crystal Habit

Euhedral (Octahedral) Red Spinel (center, and right)

10. Geode Crystal Habit

Uncut Geode (left), Amethyst Geode (center), and Quartz Geode (right)

11. Granular Mass Habit

Granular Mass Peridot (center), and Peridotite Encrustation (right)

12. Mamillary Crystal Habit

Mamillary Malachite (center), and Chalcedony (right)

13. Octahedral Crystal Habit

Octahedral White Spinel (center), Diamond Rough (right)

14. Pinacoid Crystal Habit

Pinacoid (Prismatic) Aquamarine (center), Emerald (right)

15. Striated Crystal Habit

Striated (Prismatic) Tourmaline (center), Striated Sapphire (right)

16. Tabular Crystal Habit

Tabular Calcite (center), Ruby (right)