ASS #5: The Diamond Crystal PDF

ASS #5: The Diamond Crystal The Diamond Crystal: -the term crystal to describe symmetrical mineral specimens in the early 17th century. The word comes from the Greek krystallos, which means “clear ice.” -Crystals can be organic or inorganic: they can grow in plants and animals as well as in...

ASS #5: The Diamond Crystal The Diamond Crystal: -the term crystal to describe symmetrical mineral specimens in the early 17th century. The word comes from the Greek krystallos, which means “clear ice.” -Crystals can be organic or inorganic: they can grow in plants and animals as well as in molten rock or in the laboratory. In fact, scientists have discovered that almost all solids are crystalline. Atom Structure: - Each atom has even smaller subatomic particles: protons, neutrons, and electrons. - Protons and neutrons form the nucleus, which is the center of the atom. Electrons surround the nucleus in orbits, or shells. - Different types of atoms have different numbers of shells. The number of electrons in an atom’s outer shell determines its chemical nature—its ability to combine with other atoms. - In diamond, each carbon atom shares an electron with each of its four neighboring carbon atoms. - When two carbon atoms share electrons like this, they form a covalent bond. Atoms can bond in other ways, but covalent bonds are the strongest. - Diamond’s carbon atoms are connected in groups of five, with one atom in the center and the other four surrounding it, Each outside atom is part of another group of five, these groups of five are called a tetrahedron Diamond Unit Cell: - A unit cell is the smallest group of atoms that has both the characteristic chemical composition and the basic crystal structure of the mineral, and sets standard for repeated patterns in the crystal - The core of diamond’s unit cell is formed from four tetrahedrons, held together by shared electrons in strong covalent bonds. - The carbon atoms in diamond bond under very high pressure and create a strong, interlocking atomic structure. Diamond’s atoms are closer together than the atoms of any other natural material. - Unit cells connect to build a crystal shape or crystal form, or its habit - crystal shape can indicate the atomic structure inside a crystal - diamond’s habit is most often the octahedron, a form with eight equal triangular faces. - Glassie-well-shaped, transparent diamond octahedrons with sharp, square edges (Perfectly shaped octahedral rough is so rare it often ends up as a collector’s specimen) - crystal structure or crystal lattice: the internal arrangement of repeated patterns of atoms - Crystal structure affects density, or weight per unit of volume and the closer arrangement of carbon atoms in diamond creating a larger amount with the same space in comparison to graphite,As a result, a diamond weighs about 1.6 times as much as a piece of graphite the same size. - Scientists call the relationship between weight and volume specific gravity (SG) - Almost all of the common simulants have higher SGs than diamond Cleavage Planes: - In some directions there are fewer carbon atoms, with more space between them, than in others, - These directions—planar surfaces that a mineral tends to break along due to atomic weakness—are called cleavage planes. - Cleavage is a smooth, flat break in a crystal parallel to cleavage planes. - gemologists classify crystals by their geometric properties and the symmetry of their internal crystal structures, these categories are called crystal systems - Three common crystal shapes of minerals that form in the cubic system are the cube, octahedron, and dodecahedron, in the cubic crystal system the three axes are perpendicular, - Crystal axes form a reference system for easy description of a crystal’s symmetry and shape. Crystal Systems Continued.... -the crystal structure effects the behavior of light -cubic crystals, light rays behave the same way no matter which direction they’re traveling. These crystals are described as singly refractive, or isotropic, meaning they have the same physical or optical properties in all crystal directions. Diamond Lattice Defect: - A defect is an imperfection or deviation from the ideal crystal lattice. - Other elements and minerals present in the environment the diamond crystal forms in, as well as any changes in the environment that put stress on the diamond crystal, are possible causes of defects - Three categories of defects are common in diamonds: point, line, and volume. - Coexistence of different diamond defects is common and can give rise to diamond’s unique bodycolors. - Point Defect: An imperfection or deviation of a single atom or point from the ideal arrangement of atoms in the crystal lattice - defect is created in diamond when a carbon atom is missing from its original position—creating a vacancy, this allows a trace element (or impurity elements) to come in - Nitrogen and boron are just two examples of trace elements found in diamonds. Line Defects: - Line defects, or dislocations, are imperfections or deviation from the ideal arrangement of atoms in a line in the crystal lattice. - Dislocations occur when stress is applied to the crystal lattice and produces distortion. - The plane or surface in which a dislocation travels through a crystal to cause an offset is called a glide plane. - distortion or deformation in a diamond crystal is called strain,The more dislocations that occur in a crystal, the more strain the crystal bears. Strain plays a critical role in diamond cutting. - Glide planes tend to occur along these directions and can create visible shadow-like lines known as graining, or grain lines Volume Defects: - Volume defects are imperfections or deviations from the ideal arrangement of atoms caused by three- dimensional aggregates of atoms or vacancies within the crystal lattice. Mineral inclusions and voids are examples of volume defects. - A negative crystal’s crystallographic shape always mimics the habit of the host. Classification System: - The foundation of the diamond type classification system is the presence or absence of nitrogen and boron atoms and the way they are arranged in the crystal lattice. - Understanding the system is critical when attempting to identify laboratory-grown diamonds and numerous diamond color treatments. Type 1: - In type I diamonds, nitrogen atoms replace carbon atoms in the crystal lattice, - Type Ia diamonds contain aggregated nitrogen impurities,vast majority of natural diamonds are type Ia. -“A aggregates” or “A centers” occur when nitrogen atoms are aggregated in pairs within the crystal lattice. -“B aggregates” or “B centers” occur when nitrogen atoms are aggregated in groups of four next to a vacancy in the crystal lattice. These diamonds are classified as type IaB. Mixed Type: - Most type I diamonds are a mix of more than one diamond type, Scientists believe that when diamonds form, all nitrogen impurities occur as isolated nitrogen atoms (type Ib) - When two A centers combine with a vacancy, they form a B center. This progression of nitrogen impurity aggregation can result in an almost “pure” type IaB diamond. - in many cases C centers, A centers, and B centers coexist in a single diamond crystal. The presence of both A and B centers results in a type IaAB diamond. Type 2 Diamond: - Type II diamonds include type IIa and type IIb. Both types are extremely rare. - Type IIa diamonds have no easily measurable nitrogen or boron impurities in the crystal lattice, so they are known as exceptionally pure and are generally colorless, but certain defects can cause a brown, pink, or gray bodycolor and an excellent conductor of heat - Type IIb diamonds contain boron atoms replacing carbon atoms within the crystal lattice, as well as a negligible amount of nitrogen impurities. The boron impurities cause type IIb diamonds to have blue bodycolor and to be excellent conductors of electricity. --The only way to precisely determine diamond type is by spectroscopy: the study of the interaction between matter and light. Cut Potential: - Planes along these different internal directions (are arrangement of atoms) are called crystal planes, They determine properties such as directional hardness and cleavage. - A diamond has three sets of internal crystal planes: cubic, octahedral, and dodecahedral. All diamonds, regardless of external appearance, contain all three. - Growth marks are visible features on the surface of a mineral crystal that reflect its internal growth and development - every diamond crystal has cubic planes parallel to its possible cubic crystal faces snf has both hard and soft directions. It is easier to polish, cut, and saw in the soft directions than in the hard directions. - Cubic planes have two directions they can be easily polished an sawed - Surface graining, or surface grain lines, are colorless lines or grooves on a diamond crystal’s surface. Surface graining arises when grain lines created by glide planes reach the surface. - One principle of cutting is to always polish against the grain lines instead of along them. - Internal octahedral planes—the planes parallel to the possible octahedral faces—are important because they are cleavage planes - Octahedral planes are hard and are very resistant to scratching and nearly impossible to polish - octahedral faces may also contain growth marks called trigons: triangular depressions or protrusions that occur on a diamond’s octahedral faces. - a trigon is a growth mark that is characteristic of octahedral faces. - dodecahedron crystals are fairly common in some diamond deposits. Most dodecahedrons are well suited for fashioning as round brilliants, Forms: The octahedron is the most common habit of gem-quality diamonds, but perfectly shaped octahedral rough is very rare due to how the diamonds make there way to the surface complicating the diamond crystal’s form, producing resorbed crystals, twinned crystals, aggregates, and strain. - A combination of octahedral and dodecahedral forms can be caused by resorption: the process in which the outer surface of a diamond is partially dissolved during transport. - A crystal that has gone through resorption is typically marked by smooth, rounded surfaces with indistinct features. Twinned Crystals: Some crystals consist of two or more parts formed in a symmetrical manner with shared crystal planes, these are called twinned crystals ( a result of crystal’s growth being interrupted and it begins to grow again in a different direction. - There are several distinct types of twinned diamond crystals. The most common is the macle, which looks like a flattened triangle, surface graining and trigons are seen on macles, and Macles are a challenge for diamond cutters for two reasons:sawing and polishing directions are not continuous throughout the stone. Aggregates: Aggregates are solid masses of individual randomly oriented crystals, rystals have the same internal atomic patterns, and they have either grown together or are cemented by some type of natural binding agent, Carbonado is a diamond aggregate used as an industrial abrasive. It can be black, gray, or brown. Strain: Strain in rough diamond often challenges cutters because it can make the rough more vulnerable,Strain can be seen using magnification and cross-polarized filters,presence or absence of strain is one factor that can sometimes help identify natural versus lab-grown diamonds. - Sorting begins with determinining a rough diamond is cuttable (gem or near-gem quality) or non-cuttable (industrial quality) - Industrial-quality diamond rough nearly always lacks transparency and uniform shape. Industrial diamonds can be colored or colorless, and some are cube-shaped. - Gem-quality diamond rough tends to have more uniform shape. It’s usually transparent, with a shiny to slightly rough surface Strain: Strain in rough diamond often challenges cutters because it can make the rough more vulnerable,Strain can be seen using magnification and cross-polarized filters,presence or absence of strain is one factor that can sometimes help identify natural versus lab-grown diamonds. - Sorting begins with determinining a rough diamond is cuttable (gem or near-gem quality) or non-cuttable (industrial quality) -Industrial-quality diamond rough nearly always lacks transparency and uniform shape. Industrial diamonds can be colored or colorless, and some are cube-shaped. - Gem-quality diamond rough tends to have more uniform shape. It’s usually transparent, with a shiny to slightly rough surface Diamond Shape and Value: - Shape is the single most important value factor for cuttable rough diamond, effecting how much weight the cutter can retain in the finished gem -Cutters must consider both marketability and weight retention when they decide the shape of a finished diamond - rough’s potential is described as: A makeable, or whole stone, is diamond rough that can be polished without sawing, cleaving, or splitting. -A sawable is diamond rough that will yield more weight if it’s divided to produce two stones, such as an octahedron or a dodecahedron. -A splittable, or cleavage, is diamond rough that can be divided into small, but valuable, segments by lasering or cleaving. -A flat is a shallow crystal that’s very limited in its potential shape. -Cleavage applies to a piece of diamond rough that broke cleanly along a cleavage plane.