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Oblique fault

A fault that strikes oblique to, as opposed to parallel or perpendicular to, the dominant structure

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Opal

An amorphous or microcrystalline mineral, SiO2.nH2O; commonly used as a gem or semi precious stone. Varieties from Lightning Ridge, Australia are among the highest in commercial value due to their striking colour

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Olivine

Minerals from the magnesium iron silicate group, (Mg,Fe)2SiO4

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Oligocene

An epoch following the Eocene and prior to the Miocene.

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Offset

A zone of secondary mineralization intersecting a main strike from a perpendicular or other angle

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Oxygen

A clear, diatomic gaseous element. Symbol, O

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Oxide

The compound of an element with oxygen produces an oxide of that element

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Obsidian

A dark to black coloured highly siliceous volcanic glass