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Rock and Mineral Dictionary
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Click on an underlined word for more information on that subject.
If the rock or mineral term you are looking for is not in the dictionary, please e-mail us and we'll add it.

Click on an underlined word for more information on that subject.

Obsidian

obsidian

Obsidian (also called Apache tears) is a volcanic glass that is usually black, but is occasionally red, brown, gray, green (rare), dark with "snowflakes," or even clear. This glassy, lustrous mineral is found in lava flows, and obsidian stones can be massive. Obsidian is formed when viscous lava (from volcanos) cools rapidly. Most obsidian is 70 percent silica. Obsidian has a hardness of 5 and a specific gravity of 2.35. The pin above is Mahogany (brown) obsidian.

ocean

An ocean is a vast body of salt water. Oceans cover more than three-quarters of the surface of the Earth. The oceans on Earth include the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, and the Arctic Ocean. The ocean floors are composed mostly of basalt.


oceanic plates

The crust of the Earth is broken into plates. The plates are enormous chunks of rock that float atop the soft mantle. The plates are moving at a speed that has been estimated at 1 to 10 cm per year. Oceanic plates (those that are under the ocean) are thinner, younger, and denser than continental plates. These underwater plates are about 75 kilometers thick and are made of basalt rock. They are relatively young since plate formation (seafloor spreading) occurs at the margins of oceanic plates.

oceanography

Oceanography is the science that studies the physical geography of oceans.
Soil Layers diagram
O Horizon

The top, organic layer of soil, made up mostly of leaf litter and humus (decomposed organic matter).


oiling

Oiling is a process of applying mineral oil to a stone in order to enhance it and mask inclusions, make them more transparent, and darken their color. Emeralds are frequently oiled to mask their many inclusions.


old rock turquoise

Old rock turquoise is an old Persian (Iranian) turquoise term for very high quality turquoise (sky blue, veinless turquoise that retains it color).


onyx

Onyx is a semi-precious stone that is black and white, generally arranged in layers. It is a form of agate with parallel banding. This structure lends itself to cameo making. Onyx is a species of chalcedony (microcrystalline quartz).
opal

opal

Opals are semi-precious stones that are luminous and iridescent, frequently with inclusions of many colors ("fire"). Opal is a mineral composed of noncrystalline (amorphous) silica (and some water) and is a species of quartz. There are three major types of opals: common opal, opalescent precious opal (white or black, with a rainbow-like iridescence caused by tiny crystals of cristobalite), and fire opal (a milky stone that is fiery orange to red in color with no opalescence). Contra luz opals are transparent opals that show a brilliant play of iridescence only when light shines through the stone. Many opals have a high water content - they can dry out and crack if they are not cared for well (opals should be stored in damp cotton wool). Some opals are treated with oil, wax or resin to enhance their finish. Opals have a hardness of 5.5 to 6.5 and a specific gravity of 1.98-2.50. Opals are found in many places worldwide, including Kenya, Czechoslovakia, Brazil, Peru, Honduras, Mexico, Canada, and the USA -- but Australia has a tremendous variety of beautiful opals.


opalescence

Opalescence is a milky white/blue type of iridescence.


opaque

Opaque means blocking the passage of light (as opposed to translucent or transparent).


orthoclase

Orthoclase (Potasium Aluminum Silicate or alkali feldspar or K-spar, KAlSi3O8) is named for its angles of cleavage. In Greek, orthos means "right" and kalo means "to cleave"; its two cleavages are at right angles to each other. Orthoclase is a common monoclinic crystal found worldwide, often in granite and other igneous rocks. It is also called adularia (moonstone, an oplescent variety) and orthose. This vitreous feldspar is colorless to white to pink to greenish, and is transluent to transparent. Orthoclase has a hardness of 6 and a specific gravity of 2.53-2.56. The streak is white. Orthoclase is used an an abrasive in scouring powder. When orthoclase is mixed with kaolin and quartz, the result is porcelain. You can use orthoclase to make your dishes and then wash them.

ouro verde

Ouro verde (meaning "green gold" in Portuguese) is a type of quartz crystal found in Brazil. This transparent stone is always irradiated (to give it its pale, golden-green color).

outcrop

An outcrop is a place where bedrock is visible on the surface of the Earth.

outgassing

Outgassing is the release of gases into the atmosphere during volcanic eruptions. A lot of of our atmosphere (including water vapor, nitrogen, and argon) came from outgassing.

oxidation

Oxidation is a chemical process in which oxygen atoms bond to atoms of a material (like a metal) and electrons are transferred from the oxided material to the reduced material. Iron oxidizes when exposed to air and moisture, forming iron oxide (rust). Silver oxidizes (tarnishes, turing the surface black) when it is exposed to hydrogen sulfide in the air (forming Ag2S, silver sulfide).


ozone

Ozone is a form of oxygen (O3); it consists of three connected oxygen atoms. Ozone is formed when oxygen molecules (O2) bonds are broken by high energy (bombardment by energetic electrons or high energy radiation).


ozone layer

The ozone layer is a region of the stratosphere which contains most (about 90%) of the Earth's atmospheric ozone. It is about10-25 miles (15-40 km) above the Earth's surface. The ozone layer shields the Earth from Ultraviolet B rays that come from the Sun. The ozone layer is becoming depleted, and there is an "ozone hole" over Antarctica.

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Rock and Mineral Dictionary
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Click on an underlined word for more information on that subject.
If the rock or mineral term you are looking for is not in the dictionary, please e-mail us and we'll add it.

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