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Geology and Mineralogy of Gemstones. David Turner R.Читать онлайн книгу.

Geology and Mineralogy of Gemstones - David Turner R.


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but excludes some topics like synthetic materials and coral.

      This volume aims to include an abundance of up‐to‐date scientific findings, but as the progress of research marches forward it will be inevitably somewhat out of date upon printing. Notification of significant omissions, errors, and new science will always be appreciated, as will be suggestions for new content and ways in which the book has been successfully used!

      Acknowledgements

      The book benefited greatly from constructive reviews by Fernando Corfu (University of Oslo), Matthew Field (AMEC Environment & Infrastructure UK Limited), Ian T. Graham (University of New South Wales), Stefanos Karampelas (DANAT, Bahrain Institute for Pearls & Gemstones), Aaron Palke (Gemological Institute of America), Lin Sutherland (Australian Museum), Kimberly Tait (Royal Ontario Museum), and one anonymous reviewer. All errors and omissions are, of course, the responsibility of the authors.

Part I Developing Base Knowledge

      1.1 Minerals and Rocks

      In order to understand the world of gemstones and their geological origins, we first need to build a foundation by learning about some mineralogy concepts. Mineralogy, in its most basic definition, is a branch of science concerned with the study of minerals. Though seemingly a fairly narrow topic, mineralogy reaches far into a number of other disciplines from medicine (e.g., effects of mineral fibers on lungs) to ceramics (e.g., firing of clay‐based pottery) and the origins of the Earth itself (e.g., mineral inclusions trapped in diamonds). This book focuses on understanding the geological origin of gemstones, most of which are natural materials known as minerals and are found within rocks. Before deciphering their origins, we must understand gemstones as minerals.

      A lengthy yet concise definition of the term mineral is:

      A mineral is a (i) naturally occurring, (ii) solid with a (iii) definite (but generally not fixed) chemical composition, and a (iv) crystalline structure. It is usually formed by (v) inorganic processes.

      The International Mineralogical Association lists 5,673 distinct mineral species as of early 2021 and there is active research to understand how the diversity of mineral species has changed through time (Hazen et al., 2015). Rocks can essentially be thought of as (i) naturally occurring aggregates of (ii) one or more mineral species held together through interlocking grains. It is important to note that rocks and minerals are natural solids and usually formed by inorganic processes. Materials made by people in a laboratory or elsewhere, such as synthetic crystals, would therefore not qualify as rocks or minerals though may share some similar properties. Since most gems are minerals, this clearly defined way of looking at minerals allows us to easily differentiate a valuable ruby gemstone from common red glass.

      Gemstones earn their value primarily through subjective factors, including historical and cultural values, as well as through marketing and influence. Objective factors that contribute to a gemstone’s value include its hardness, toughness, and rarity, and gemmological factors like clarity, size (measured in “carats”, where 1 carat = 0.2 grams), and the cut of a specific gemstone itself. Combined, these factors suggest that as world supplies of gemstones ebb and flow and cultures change, so can the value of all gemstones. Nevertheless, some gemstones retain their importance. Geologist Sydney H. Ball provided an insightful quote from his 1935 article titled “A Historical Study of Precious Stone Valuations and Prices” that is still valid today, over 80 years later:

      Exceptionally fine gems are so rare that they have no fixed price, and each transaction becomes a matter of negotiation between buyer and seller. As with a fine painting or other work of art, set rules do not hold. Such are red, green, or blue diamonds, white diamonds of unusual size and brilliancy, rubies of over four carats, emeralds of fine deep color and relatively free of flaws, particularly if of good size, and unusually fine sapphires.

       (Ball, 1935)

      Synthetic crystals are not considered minerals but can be considered as gemstones; however, their trade names should always be preceded by the word “synthetic” or an equivalent term. The acceptance of synthetic crystals in the gem trade is mixed, with some people rejecting human‐made materials altogether and others arguing they are just as valid as natural gemstones. The middle ground is occupied by most professionals and they accept synthetics as gemstones so long as they are designated as such (e.g., “synthetic ruby” or “laboratory‐created emerald”). Imitations, on the other hand, are inexpensive materials that are designed with the intent to mimic a gemstone of greater value. A range of treatments can also be applied to many gemstones. Treatments can be as simple as heating to influence the overall color to more sophisticated recipes, including subjecting gemstones to complex heating, irradiation, and pressure conditions in a specific order to achieve specific results.

      All matter is composed of particles known as atoms. Atoms are made up of protons, neutrons, and electrons. Elements are collections of these particles (i.e., an atom) and their identity is defined by the number of protons. Atoms can combine through chemical bonding to form specific chemical compounds. Minerals are collections of atoms arranged and bonded together in specific ways.

      1.3.1 Protons, Neutrons, and Electrons

      1.3.2 Atomic Mass

      The total number of neutrons and protons defines the atomic mass of an atom. Because electrons are so small, they do not contribute much to the overall atomic mass of an atom. The weights of atoms are given in atomic mass units, or amu, where both protons and neutrons have an atomic mass approximately equal to 1 amu. Helium has two protons and almost always two neutrons; its atomic mass therefore is 4 amu.


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