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Oamaru stone is a kind of limestone, of building quality, found at Oamaru on South Island, New Zealand.
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Obsidian is an extrusive igneous rock that is composed mainly of the minerals feldspar and quartz. It has been used as a natural form of glass for its cutting properties since prehistoric times - being known as volcanic glass - and today is used as a gemstone.
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The Oligocene was the fifteenth geological period, 42,000,000 years ago.
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Oligoclase has the formulae (Na,Ca)AlSi3O8 and a relative hardness of 7. It is a plagioclase feldspar. It is of interest to petrologists and collectors. Varieties: labradorite, anorthite.
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Olivenite is a rare olive-green, brown or yellow coloured secondary mineral found in deeply weathered, highly concentrated copper sulphide ore bodies. It is a hydrous arsenate of copper confirmed as a distinct species in 1820 and sometimes mined as a copper ore. Olivenite has a relative hardness of 3.
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Olivine is a group of minerals characterised by their green colour, the typical
olivine has the formulae (Mg,Fe)2SiO4 and a relative hardness of 7. It's name is derived from the characteristic olive green colour. A common rock forming mineral. A clear green variety is called peridot and has some uses as a gem.
Olivine minerals are hard and resistant to heat, and are used as abrasives and refractory sands.
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Onyx (named on account it resembles the colour of a fingernail) is a variety of chalcedony, similar to agate but featuring straight, rather than curved bands.
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Opal is an amorphous, or non-crystalline form of silica with the formulae SiO2ùnH2O and a relative hardness of 6. It is always a secondary mineral, found lining and filling cavities in igneous and sedimentary rocks where it has been deposited by hot waters. The ordinary varieties are common but the precious varieties are quite valuable. Opal is also a petrifying agent which may permeate fossil wood, corals, shells etc, preserving even the minute structure of the organic remains. It was discovered in 1964 that opal is made up of close-packed silica spheres, and in 1974 Pierre Gilson produced the first synthetic opal.
In folklore, opals are considered unlucky. They are full of 'eyes' which interfere with the sanctity of the privacy of the wearer or home.
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The Ordovician period was the fourth geological period, 375,000,000 years ago.
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Orpiment is a secondary mineral with the formulae As2S3 and a relative hardness of 2. It is a rare mineral usually associated with realgar. Orpiment is an important source of industrial arsenic and is used in dyeing. Orpiment can be distinguished from sulphur by its perfect cleavage, and strong smell of garlic (arsenic) when heated.
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Orthoclase has the formulae KAlSi3O8 and a relative hardness of 6. It is used in the manufacture of porcelain and for other industrial purposes.
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Orthorhombic refers to a rectangular crystal with three axes of different lengths and all at right angles to each other. A closed book is a basic example.
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Oxford clay or the Oxfordian beds, are the lowest subdivision of the Upper Jurassic rocks. Their outcrop stretches from Weymouth, past Swindon, Oxford and Bedford, into North Yorkshire. In its typical development it is 150 meters or more in thickness, blue or grey in colour, and contains thin layers of limestone and many septarian nodules.
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Oxide refers to a group of minerals where oxygen joined with a metal is a major constituent.
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Oxidized zone refers to the portion of an ore body that has been altered by downward percolating groundwater and which contains dissolved oxygen and carbon dioxide.
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Ozocerite is a wax-like mineral resin. It is sometimes called native paraffin, or mineral wax.
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Abbreviations
Rocks & Minerals
