ASS #6 Diamond & Color PDF

ASS #6: Diamond and Color Nature of Light: - Light is a form of radiant energy or radiation. It’s a combination of electric and magnetic (electromagnetic) energies, and it travels in waves. - A wavelength is the distance between two consecutive high or low points along an energy wave’s path. Wavele...

ASS #6: Diamond and Color Nature of Light: - Light is a form of radiant energy or radiation. It’s a combination of electric and magnetic (electromagnetic) energies, and it travels in waves. - A wavelength is the distance between two consecutive high or low points along an energy wave’s path. Wavelengths are measured in nanometers, abbreviated nm. - A nanometer is a tiny unit: one nm is equal to a millionth of a millimeter. - Light rays with shorter wavelengths have higher energy, while rays with longer wavelengths have lower energy. - The human eye can detect only visible light, which is a small portion of the electromagnetic spectrum. - Electromagnetic spectrum is the range of wavelengths of radiant energy extending from high to low energy. - Visible light extends from about 400 nm (violet wavelengths) to about 700 nm (red wavelengths) in the spectrum, this wavelength range is the visible spectrum. Selective Absorption: - White light is the combination of the colors in the visible spectrum: red, orange, yellow, green, blue, and violet. - When visible light meets a transparent faceted gem material, some of it reflects off of the stone’s surface. The rest enters the stone, where the gem’s crystal structure absorbs some of the color components of white light, this process is called selective absorption - The color components not absorbed by the gem return to your eye via a process called transmission—and present as the gem’s color. - Diamond’s color is primarily caused by imperfections or defects in its crystal lattice, - Color Centers: are the defects that cause color, but not all defects in diamonds cause color Absorption Spectrum: -The spectroscope splits white light into its spectral colors. -Absorption Spectrum: The dark areas show which spectral colors the diamond absorbs, and the bright areas show the components that are transmitted. -spectroscope splits white light into its spectral colors. -A more powerful instrument called a spectrometer which can help gemologists see the exact wavelengths a diamond absorbs and transmits by recording a linear graph. Atoms, Electrons, and Energy Levels: -Electrons play an especially important role in selective absorption because visible light interacts primarily with them. -electron absorbs a specific wavelength of visible light, it provides energy to allow the electron to rise temporarily to a higher level called an excited state, and when electron releases that energy when it returns to its ground state, this called transition The Band Structure of Diamond: -Electronic band structure (or band theory) is used to explain the properties of solid materials. -Electrons in a lower-energy band called a valence band move toward a higher-energy level called a conduction band. -A constant energy difference—called the band gap—separates the valence band from the conduction band. -All solid materials are divided into three groups based on their band structure: insulator, semiconductor, and conductor. -a defect-free diamond is an insulator, so diamonds that lack color centers are colorless because they do not absorb visible light. -Defects work as newly added “steps” in the band gap. Instead of crossing the whole gap, electrons can make transitions between the conduction or valence band and these steps, or even in between these steps. -Nitrogen and boron impurities are typical examples of defects that cause color in diamonds The Band Structure of Diamond continued.... - Electrons absorb some visible light to obtain enough energy to travel from the nitrogen step to the conduction band, causing a yellow color in the diamond - Similarly, when boron replaces carbon in the crystal lattice, a step is created, electrons absorb some visible light to jump from the valence band to the boron step and color the diamond blue. Causes of Color In Diamond: - The same defect can cause different colors based on concentration and presence of other defects. For example, a radiation-generated defect can create green or blue depending on the diamond’s nitrogen content. Naming Defects: - Some defect names are self-explanatory: they describe the defect’s cause or nature. For example, the defect and color center caused by irradiation is named “GR1,” in which “GR” represents “general radiation.” -These names were based on whether the defects were originally detected in the absorption spectra of natural (N1, N2, N3, etc.) or heated (H1, H2, H3, etc.) diamonds. Yellow and Orange: - Yellow diamonds are some of the most commonly encountered natural-color diamonds, which a is mainly a result of nitrogen - nitrogen absorbs light in the blue end of the spectrum and results in a yellow bodycolor, making it type 1 - Type 1a diamonds are a result of a N3 color center,consists of three nitrogen atoms adjacent to a vacancy, causing absorption in the blue region, rest of the visible light is transmitted creating yellow bodycolor. - The H3 color center (two nitrogen atoms adjacent to a vacancy) can also cause yellow bodycolor. -Natural yellow type la diamonds colored by the N3 defect are often called “cape” in the trade. - less common, cause of yellow color is isolated nitrogen atoms (C centers) causing absorption within the ultraviolet region that extends into the blue end of the visible spectrum, producing a highly saturated yellow or orange-yellow, often described as “canary” diamonds -Type Ib diamonds usually contain smaller concentrations of nitrogen impurities than type Ia diamonds, - A higher concentration of isolated nitrogen impurities may cause an orangy bodycolor. -The "Pumpkin Diamond" 5.54 ct and the "The Orange" 14.82ct are the largest diamonds ever found in that color range Green: -Most are yellow-green or pure green; others are dominant green mixed with blue, brown, or gray. -Green stones with saturated colors are some of the rarest and most sought after. - green color in a diamond is exposure to radioactive minerals and fluids in the earth’s crust over time. -“General Radiation 1” or GR1 defects, absorb red parts of visible light, diamonds naturally colored by the GR1 defect are typically pure green or blue-green -Most green diamonds colored by this defect have a higher number of nitrogen-related defects, whereas green-blue to blue diamonds colored by this defect typically contain a lower number of nitrogen-related defects. - naturally irradiated diamonds have areas of green or brown radiation damage on their surfaces, seen as “spots” or patches of color, and referred to as staining. -Radiation stains can be caused by direct contact with radioactive minerals, often in kimberlite host rock. -Radiation stains appear green to brownish in most diamonds. - Some defects that cause green color in diamond involve hydrogen and nickel impurities. -Hydrogen-related defects cause absorptions at the red end of the visible spectrum and beyond -Nickel-related defects that cause green color are less common due to the large size of the nickel atom that must be accommodated in the lattice. -H3, hydrogen, and nickel defects are associated with nitrogen impurities in the diamond lattice; therefore green diamonds colored by these defects are all type Ia. -The " Dreseden Green" 41ct, is a rare, uniform green color is caused by exposure to radiation over a long time span. -"Chameleon: diamonds when heated or left in the dark for an extended period of time (days to weeks), they temporarily lose their green color and change to yellow or orangy hues, nature of the color change is unclear Blue, Violet and Gray: - Blue diamonds are among the most famous and rare gemstones. They generally have a slight hint of gray, so their color is never as highly saturated as blue sapphires. -Violet and gray diamonds coexist on a color continuum with blue diamonds. -diamonds with a pure blue color are type IIb. Their color is caused by boron atoms that replace carbon atoms in the crystal lattice. -The hydrogen-related defect can also cause blue, violet, and gray. - green-blue to blue diamonds colored by this GR1 color center defect typically contain a lower number of nitrogen-related defects. -The Hope is an important historic diamond. The original ancestor of the Hope diamond was an approximately 115-ct. stone, cut into 69 ct triangular diamond called the Diamant Bleu de la Couronne by Louis XIV, Twenty years later, a 45.52-ct. blue diamond appeared for sale in London and eventually became part of the collection of Henry Philip Hope. Pink, Red, Purple and Brown: - Pink and red diamonds are among the most valuable due to their attractiveness and extreme rarity. -Pure red and pure purple diamonds are the rarest of all natural diamonds. - The main cause of pink-to-red, purple, and brown colors in diamonds is an absorption band centered at approximately 550 nm in the visible spectrum,correlated with deformation, -The color in many pink, red, purple, and brown diamonds is often concentrated within parallel narrow bands called colored lamellae: closely spaced grain lines resulting from deformation. -On rare occasions, a single nitrogen atom associated with a vacancy (NV defect) colors a natural pink -Natural pink diamonds colored by the NV defect are extremely rare; however, NV defects are almost exclusively the cause of color in treated and lab-grown pink diamonds. -Brown diamonds are quite common. The vast majority have color due to vacancy clusters generated by deformation. - The 5.11-ct. Moussaieff Red was the largest Fancy red diamond ever documented by GIA at the time of grading in 1997. Black,Gray and White: -The most famous Fancy black diamond is the 67.50-ct. Black Orlov. -Fancy white diamonds should not be confused w/colorless diamonds that are often called “white” within the trade. - black and white diamonds are usually colored by volume defects, such as internal inclusions that reduce the transparency giving range from transparent to opaque but are generally translucent to opaque. -Natural black diamonds are usually colored by dark inclusions of graphite, sulfide, magnetite, hematite, or other iron-bearing minerals. - Black diamonds usually contain a mass of dark crystal inclusions, while abundant micro-inclusions or cloud-like features usually cause a grayish appearance. -white diamonds are sometimes referred to as “opalescent” due to the unusual way that light scatters inside the stones. The terms “hazy,” “milky,” or “cloudy” are often used to describe the appearance when observed through a microscope. Luminescence: - Energy from a light source (e.g., visible light, ultraviolet light, or X-rays) can knock electrons out of their stable ground state and raise them to an excited state,This energy releases as heat and/or visible light that is perceived as color. When visible light is emitted, it is called luminescence. -Fluorescence is commonly described as a form of luminescence observed from a gemstone during exposure to UV or X-ray radiation. -Phosphorescence occurs when the gem continues to emit light after the UV or X-ray stimulation is removed.The eye can only detect phosphorescence with a lifetime of about 1 second or longer. -the light emitted by the gem always has lower energy than the light it absorbs. - About 35 percent of colorless natural diamonds emit fluorescence to UV radiation, Defects can produce luminescence in diamond - The vast majority of these diamonds show blue fluorescence caused by the N3 defect (three nitrogen atoms and a vacancy). Assessment and Application: -Fluorescence and phosphorescence assessment can be done by visual observation or with instrumentation. -Luminescence is a tool for diamond identification because it indicates hidden defects in the crystal lattice.