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Obelisk = it is though that the name is derived from the Greek obeliskos, meaning "a small spit".
Obsidian = very hard volcanic glass used since millennia for tools. When found in its natural state in nodules it can be dated by measuring the thickness of its hydrated layer on the surface.
Overburden = the earth and waste stone that has to be removed before a quarry can expose good stone.
Open pit quarry = contrasted with the underground or 'mine' style of quarry.
Ogham (ogam) = ancient alphabet, in which letters are formed of parallel lines which meet or cross a base-line. Possibly of Irish origin.
Orthostat = large stone or slab, set vertically in a structure.
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