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It is hoped that this article will give the gemologist a ready way to convey how nature first created the rough diamonds.For the reader who is unfamiliar with geologic terms, a glossary is presented at the end of the article.This graph depicts the rise of temperature with depth (the geothermal gradient) in the lithosphere.Diamonds are stable under the high pressure and temperature conditions that are only met at great depth in the earth’s mantle.Carbon in the earth can occur in oxidized forms, such as when bound with oxygen in CO, or in reduced forms such as diamond, graphite, or bound with hydrogen in methane and other organic molecules.The experimentally determined pressure-temperature conditions where diamond is stable (figure 3) dictate formation pressures higher than 40 thousand atmospheres (4 GPa) and temperatures of 950–1400ºC.Terms listed in the glossary are italicized on their first use in the text. Carbon is widely dissolved in the earth’s silicate minerals at part-per-million levels and lower.But whenever carbon occurs as a free species, diamonds have the potential to form.
Within the earth, temperature always rises with depth along a path known as the Figure 3.Rubidium-87 comprises 27.85 percent of the total atomic abundance of rubidium, and of the four isotopes of strontium, only strontium-87 is formed by its decay.The method is applicable to very old rocks because the transformation is extremely slow: the half-life, or time required for half the initial quantity of rubidium-87 to disappear, is approximately 50 billion years.Recent work of the research community (summarized in Pearson and Shirey, 1999; Cartigny, 2005; Harlow and Davies, 2005; Stachel et al., 2005, 2009; Stachel and Harris, 2008, 2009; Gurney et al., 2010; Shirey et al., 2013) has been of considerable interest to economic geologists searching for natural diamonds, guiding their models of how to explore for new occurrences.But it is also of importance to the practicing gemologist, since these are fundamental questions that a wearer of a beautiful diamond might ask.
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Only a subset of these localities are rich enough to be mined for diamonds.