EnchantedLearning.com Rock and Mineral Dictionary

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.

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 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.

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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