Ass #5: Key Terms & Concepts PDF

Ass #5: Key Terms Aggregate—A solid mass of individual randomly oriented crystals, intergrown or held together by a natural binding agent. *Atom—The basic structural unit of all matter. *Cleavage—A smooth, flat break in a crystal parallel to a cleavage plane. Cleavage Plane—A...

Ass #5: Key Terms Aggregate—A solid mass of individual randomly oriented crystals, intergrown or held together by a natural binding agent. *Atom—The basic structural unit of all matter. *Cleavage—A smooth, flat break in a crystal parallel to a cleavage plane. Cleavage Plane—A planar surface along which a mineral tends to break due to atomic weakness. Covalent Bond—A chemical bond formed by two atoms sharing electrons. Crystal—Solid matter with atoms arranged in a regular, repeating pattern. Crystal Axes—A reference system used to describe the symmetry and shape of crystals. Crystal Planes—Any set of parallel and evenly spaced planes distributed along certain crystal directions in which atoms are more closely packed. Crystal Shape (form)—The geometric shape of a well-formed crystal. Crystal Structure (lattice)—The regular, repeating arrangement of atoms in a mineral. Crystal Systems—Categories of crystals based on their symmetry and internal structure. *Crystalline—Composed of crystals or related to crystals. Defect—An imperfection or deviation from the ideal crystal lattice. *Doubly Refractive (anisotropic)—Possessing different physical or optical properties in different crystal directions. Glassie—A well-shaped, transparent octahedral diamond crystal with sharp, square edges. *Glide Plane—The plane or surface in which a dislocation travels through the crystal to cause an offset. *Graining (Grain Lines)—Visible shadow-like lines in a diamond caused by glide planes in the crystal structure. Growth Marks—Visible features on the surface of a mineral crystal that reflect its internal growth and development. *Habit—The characteristic crystal shape or form of a mineral. Line Defect (Dislocation)—Imperfection or deviation from the ideal arrangement of atoms in relation to a line in the crystal lattice. Macle—A flat, triangular twinned diamond crystal. Octahedron—A crystal shape with eight equal triangular faces. Point Defect—An imperfection or deviation from the ideal arrangement of atoms in relation to a single atom or point in the crystal lattice. Resorption—The process in which the outer surface of a diamond is partially dissolved during transport. Singly Refractive (Isotropic)—Possessing the same physical or optical properties in all crystal directions. *Specific Gravity (SG)—The ratio of the weight of a material to the weight of an equal volume of water. Spectroscopy—The study of the interaction between matter and light. Strain—Distortion or deformation in a crystal. *Stress—The force required to produce distortion in the crystal lattice. Surface Graining—Colorless lines or grooves on a diamond crystal’s surface. Tetrahedron—In diamond, a group of five carbon atoms with one at the center. Trace Elements—Atoms in a gem that aren’t part of its essential chemical composition. *Trigon—A triangular depression or protrusion that occurs on a diamond’s octahedral faces. Twinned Crystal—Crystal consisting of two or more parts formed in a symmetrical manner with shared crystal planes. *Unit Cell—The smallest group of atoms with the characteristic chemical composition and the basic crystal structure of a mineral. *Vacancy—A point defect created when an atom is missing from its original position in the crystal lattice. *Volume Defect—Imperfection or deviation from the ideal arrangement of atoms caused by a three- dimensional aggregate of atoms or vacancies within the crystal lattice. Key Concepts: Covalent bonds are the strongest of all atomic bonds. Diamond’s atoms are more closely bonded than the atoms of any other material. The most common habit of a gem diamond is the octahedron, but perfectly shaped octahedral rough is rare. Defects in diamond are common and not always undesirable. Three categories of defects are common in diamonds: point, line, and volume. *The more dislocations that occur in a crystal, the more strain the crystal bears. Mineral inclusions and voids are examples of volume defects. A negative crystal’s crystallographic shape always mimics the habit of the host. 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. Type I diamonds have nitrogen atoms replacing carbon atoms in their crystal lattice. Most type I diamonds are a mix of more than one diamond type. *Type IIa diamonds have no easily measurable nitrogen or boron impurities in the crystal lattice. *Type IIb diamonds contain boron atoms replacing carbon atoms within the crystal lattice, as well as a negligible amount of nitrogen impurities. The most reliable and precise determination of diamond type is by spectroscopy. All diamonds, regardless of their external appearance, contain cubic, octahedral, and dodecahedral crystal planes. Cubic planes provide the most efficient sawing directions for dividing large, regularly shaped crystals. It is easier to polish against surface grain lines than along them. Diamond’s octahedral planes are cleavage planes. A trigon indicates an octahedral plane in a diamond. A combination of octahedral and dodecahedral forms can be caused by resorption. The macle is the most common twinned diamond crystal. Shape is the single most important value factor for cuttable rough diamond.