Gemmology, Third Edition by Peter Read C Eng MIEE FGA DGA

By Peter Read C Eng MIEE FGA DGA

Because Gemmology was once first released, millions of readers have used it to realize an in-depth appreciation of the technological know-how of gemmology, its background and perform. This 3rd variation has been revised to hide, specifically, This 3rd version has been revised to hide, specifically, new HPHT diamond enhancement tools and beryllium lattice diffusions innovations for corundum in addition to the low strain hot temperature CVD synthesis of thick movie diamond

It contains info of the Gemmological institutions lately brought beginning path, and gives exam notes.

* includes crucial info for the Gem-A exams
* totally up-to-date to hide contemporary advances and instruments
* a useful source for gemmologists around the globe

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Extra resources for Gemmology, Third Edition

Example text

4. (There are usually several possible axes of symmetry in a crystal; these are described as two-fold, three-fold, four-fold or six-fold axes. 4 From left to right, examples of two-fold, three-fold and four-fold axes of symmetry in a cubic crystal. 5). A cube has nine such planes. 5 Showing two of the planes of symmetry in an octahedron Centre of symmetry A crystal is said to possess a centre of symmetry when identical faces and edges occur on exactly opposite sides of a central point. The seven crystal systems and their elements of symmetry As mentioned earlier, crystals can be grouped into seven systems.

Fluorite and calcite have perfect cleavage and are also easy to cleave. Perfect cleavage can be recognized by the presence of a series of very shallow steps where splitting has occurred along different but parallel layers of atoms. Descriptions of gemstone cleavages in terms of quality and direction are included in the gem profiles in Appendix C at the rear of this book. Lapidary problems caused by cleavage planes When polishing a gemstone which has perfect or good cleavage, the lapidary must ensure that none of the facets lie exactly parallel to a cleavage plane.

When sudden cooling occurs, there is not enough time for the islands and chains to become reorganized into a largescale crystalline structure. The solidified material then has an internal structure having only tiny scattered regions of crystalline order. Glass is a material of this type containing frameworks of disordered silicon tetrahedra. Because of the lack of overall crystal structure, glass cannot exhibit any of the crystalline characteristics such as external shape, or cleavage and pleochroism (subjects which are covered in Chapters 5 and 10).

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