 | | Journal (article/letter/editorial) | | THE GEM VARIETIES OF OPAL By RALPH C. GOSSE Albany, New York Introduction This paper is by no means...the many varieties, occurrences, colors, of the opal. Those only of gem quality will be treated here...groups, precious and common. Precious Opal: Any variety of the opal showing a play of color, regardless..."precious" used in connection with opal does not necessarily infer value, an opal may be of the precious variety...not gem quality, and vice versa. Common Opal: Any variety of opal which has no play of color, regardless |  | | Journal (article/letter/editorial) | | (2006) 67 – 86 www.elsevier.com/locate/chemgeo Opal as a U–Pb geochronometer: Search for a standard...data for several samples of precious and common opal, which we tested as potential standards for ion...determinations. In contrast, all studied samples of common opal are enriched in uranium and contain sufficiently...Pb in some of the opals, and the presence of two opal generations of different ages in one of the samples...for a better opal standard. © 2006 Elsevier B.V. All rights reserved. Keywords: Silica; Opal; Chalcedony; |  | | Book (edition) | | the outstanding sho,vs of the annual San Diego County Fair at Del Mar, California. In addition, the collections...(chalcedony replacing wood, shell, coral, or bone), opal replacing wood. R eplacements of O ther Minerals...AND CLEAVAGE SU ITE Conchoidal Fracture Co1nmon opal, obsidian, quartz. Granular Fracture Marble, granite...with a small hammer. Very brittle minerals include opal, sulfur, halite, calcite, and galena, MINERALOGY...chalcedony, agate or jasper after wood, aventurine), opal (common), orthoclase (twin), rnicrocline (perthite) |
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