DDG Ass #16 Key Terms and Concepts PDF

DDG Ass #16 Key Terms and Concepts: Catalyst—A substance that increases the rate of a chemical reaction. Crossed Polarizers—Two polarizers with their light-transmitting directions aligned perpendicularly so light cannot transmit through them. Cuboctahedron—A crystal with both octahedral...

DDG Ass #16 Key Terms and Concepts: Catalyst—A substance that increases the rate of a chemical reaction. Crossed Polarizers—Two polarizers with their light-transmitting directions aligned perpendicularly so light cannot transmit through them. Cuboctahedron—A crystal with both octahedral and cubic faces. Growth Sector—A three-dimensional region with a common crystallographic growth plane. Interference Colors—Spectral colors caused by the interaction of two light rays seen under cross- polarized light. Laboratory-Grown (lab-grown) Diamonds—Manufactured diamond with essentially the same physical, chemical, and optical properties as natural diamond. Metastable—Theoretically unstable but regarded as stable due to its longevity. Polarizer—A plastic disk embedded with specifically oriented microscopic crystals, designed to transmit polarized light. Substrate—A template or seed for new CVD diamond growth. Key Concepts: The two major commercial technologies for synthesizing lab-grown diamonds are the high-pressure, high- temperature (HPHT) and chemical vapor deposition (CVD) methods. Diamond synthesis research began before 1800, but diamonds weren’t grown in the lab until the 1950s. High production costs limited the use of lab-grown diamonds in jewelry from the 1990s to the early 2000s. The lab-grown diamonds used in jewelry before 2010 were primarily colored and produced in small sizes. Lab-grown diamond producers must consider factors such as equipment capacity, growth time, and costs. HPHT diamonds are grown in conditions that replicate the earth’s natural process. HPHT lab-grown diamonds are produced by converting a solid carbon source to diamond with an HPHT press. Growing diamonds with the HPHT process requires a carbon source, heating element, metal catalyst, seed crystal, and temperature gradient. HPHT diamond crystal growth is predominantly cuboctahedral. Isolated nitrogen is extremely difficult to remove from HPHT lab-grown diamonds. Colorless HPHT lab-grown diamonds are type IIa or weak type IIb. The only internal inclusions an HPHT lab-grown diamond might contain are remnants of the metal catalyst it grew in. The interference colors in diamonds reflect the intensity of the strain. HPHT lab-grown diamonds never show any clear strain patterns. HPHT lab-grown diamonds generally show a stronger fluorescence reaction to shortwave UV than to longwave UV. Blue and colorless HPHT lab-grown diamonds often show green-blue phosphorescence. CVD diamond growth occurs at very low pressures and moderate temperatures. CVD diamonds are grown by converting a gaseous carbon source to diamond using a reactor. Growing diamonds using the CVD process requires a gaseous carbon source, hydrogen gas, a substrate, an energy source, and a heating element. CVD diamonds are grown layer by layer and consist of a series of steps, or growth sectors, known as terraces and risers. CVD diamond rough resembles slabs with a square cross section. The majority of CVD lab-grown diamonds are type IIa. The appearance of many colorless and colored CVD lab-grown diamonds is due to post-growth treatment. Most CVD lab-grown diamonds cannot be identified using standard gem-testing equipment and must be submitted to a laboratory for identification. It is extremely rare for CVD lab-grown diamonds to fluoresce under longwave UV. The characteristic terrace and riser growth sectors in CVD lab-grown diamonds can be seen with a DiamondView. Most diamond screening devices are based on the fact that the majority of natural diamonds are type Ia. If you are not certain of a stone’s identity, submit it to a gemological laboratory for identification. Lab-grown diamonds undergo the same rigorous grading process as natural diamonds.