Study Guide for Midterm Exam 2.doc

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Earth Sciences
Cristina Millan

Earth Sciences 1108 Gemstones Study Guide for Midterm Exam 1 Spring 2014 General information for the exam: a) The format of the exam will be multiple choice, true/false, and matching. b) The test will cover material discussed/shown in class, which includes: discussions in class, powerpoints, your class notes, Schumman (textbook) readings, video clips and videos shown in class, and the pertinent eReserves (pdf) readings from Hurlbut and Kammerling The following questions are to get you thinking about the material. REMEMBER, THIS IS JUST A GUIDE!!!! INTERACTIONS OF LIGHT AND GEMS Compare and contrast color vs luster  Color: we see color when a material selectively absorbs specific wavelengths of visible light  Luster: external reflection of light in gems surface o Depends on refractive index of each stone o The higher the RI the higher the luster o Lust in not measurable, so described with names What are the different luster names? How each of those looks like or feels like? How is luster related to refractive index (RI)? 1) Metallic High RI Adamantine Vitreous Med-High RI Greasy Pearly Silky Med RI Waxy Resinous Low RI Understand transparency vs included stones. How well each of these is suited to be a faceted stone vs are cabochon? Why?  Transparency: how well light passes through a stone o Limits?  Inclusions, gas/liquid bubbles, fractures or fissures, grainy or fibrous growths within a crystal  Opaque would most likely be a cabochon, where as a transparent stone would likely be faceted to get sparkle/shine Understand reflection and refraction of light and how they are related to color, luster and sparkle  Reflection o Incident ray  reflected ray o Angle of incidence = angle of reflection o Has to do with luster  Refraction o Incident angle > refracted angle o Has to do with color and sparkle What happens to a ray of light when it shines on an object (a crystal)?  Light is reflected in every facet of a stone  Part of the light is reflected  Part is absorbed and refracted Know how to draw a diagram of a light ray hitting the surface of a crystal. Be sure to show all the angles the light ray makes with the surfaces. Know all the parts of this diagram, ray names, angle names, etc… What is the normal axis in a stone facet? Is it always the same in each facet of a stone? What does light do when going from a less dense material (air) to a denser material (glass)? Why? What happens when light goes from a denser to a less dense material?  Less dense  denser = slow down, decrease in velocity What is refractive index (RI)? What do we use it for? How/why is it useful for gem ID? -Extremely important for gem ID -Accurate -Non-destructive -Can be used in mounted stones -RI is constant for each specific gem!!! How are refractive index, the speed of light in air and the speed of light in a stone related?  RI= speed of light in air/speed of light in a stone What is a refractometer? How does it look like? What do we see when we put a stone in a refractometer?  The tool we use to measure RI on gems  It looks like a f-ed up microscope  We see where the light refracts on a scale What are the max and min refractive indices numbers in a stone?  1.4-3.2 What is a double refractive gem?  They have more than one refractive index ISOTROPIC AND ANISOTROPIC MATERIALS Understand isotropic vs anisotropic definitions. What are the differences?  Isotropic  light moves through the crystal with the same velocity in all crystallographic directions o Ex: Halite  Anisotropic  the velocity of light varies with crystallographic direction o Ex: all crystals that are not isometric; calcite Which crystals systems are isotropic and which ones arent’t?  Isotropic  cubic  Anisotropic  hexagonal, tetragonal Why causes a crystal system to be isotropic or anisotropic?  Isotropic  singly refractive gems o Light behaves the same in all directions o Isometric  garnet, diamond, spinal. o Amorphous  imitation gems such as glass or plastic; organic materials such as natural glass  Anisotropic  the velocity of light varies with crystallographic direction o Light entering anisotropic crystals splits into two mutually perpendicular planes o Anisotropic crystal What do you see when you look through an isotropic crystal? How about when you look through an anisotropic crystal?  One line in isotropic  2 lines in anisotropic How is the velocity of light different in one vs other?  In SR, the velocity of light is the same throughout  In DR, the velocity varies depending on the direction What is dispersion (aka fire)? What causes dispersion?  Dispersion  phenomenon of different amount of bending of different wavelengths of light o Rainbow-like flashes of color in faceted gems o Prism effect  when light passes from air into gems the velocity varies for each color, because each color has a different wavelength o Results from the cut of the stone (proper facet angles) o HAPPENS ON EACH FACET, NOT ON TABLE What are other optical phenomena? Compare and contrast the causes of asterism, chatoyancy, and interference effects (labradorescence, opalescence, iridescence…). How are these different/same?  Asterism o Caused by reflection of light from silky, fibrous or needle like o These included crystals show a preferred orientation within the gem o Ex: star sapphire  Chatoyance o Caused by reflection of light from silky, fibrous or needle like crystals that make up the gem o Light reflects off the fibers strongly when a light source is perpendicular to the fibers long dimension o Ex: quartz, tiger’s eye, alexandrite  Asterism vs. Chatoyance o Asterism: included minerals; seen as stars o Chatoyancy: minerals that make up the crystal will appear to move when a point light source moves across the gem surface  Interference effects o Opalescence play of colors, light is scattered by layers the structure of silica spheres in opal o Iridescence (labradorescence)  light is scattered by the intergrowth of more than one mineral in gemstones, caused by distortions in the internal arrangement of atoms, usually in metallic blue and green hues  Example: moonstone (6-6.5) Know examples of gems/minerals/rocks that display these characteristics (focus on the ones in your slides we talked about in class) SINGLE & DOUBLE REFRACTION. POLARIZED LIGHT What is a Single Refractive (SR) gem?  Light behaves the same in all directions in such isotropic materials
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