Obsidian: 
Etymology : Latin expression Obsidianus lapis, so named, according to Pliny, after one Obsidius, who discovered it in Ethiopia: compare to French obsidiane, obsidienne.
Obsidian was used in precolumbian civilization.
Aztecs used obsidian as the weapons and ustensils of choice for anything that needed to be cut.
In the Maya civilization, obsidian was coming from La Joya, about 30 kilometers from Guatemal City.
Obsidian is a natural glass. It is formed from lava that has cooled too quickly to crystallize. It is amorphous, without cleavage. The fracture is conchoidal.
Its most common color is black, obsidian can also be found in light brown, brown mottled with black, and black with a beautiful golden or silvery sheen..
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Oligoclase:
Discovered in 1826 by the mineralogist Johann August Friedrich Breithaupt .
Etymolgy: from Greek "Oligos" = "a few" and German "Klass" = "broken".
Oligoclase is a rock-forming mineral belonging to the plagioclase feldspars. In chemical composition and in its crystallographic and physical characters it is intermediate between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8). The albite : anorthite molar ratio ranges from 90 : 10 to 70 : 30.
Oligoclase is a high sodium feldspar crystallizing in the triclinic system.
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Onyx:
Etymology: from Greek "Onux" = meaning "claw" or "fingernail".
Black Onyx: its color is probably coming from a natural irradiation.
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Opal:
Etymology: from Sanskrit "opali"
Orthoclase: 
Etymology : from Greek « orthos » = straight, perpendicular.
From Johann August Friedrich Breithaupt, in 1823.
Orthoclase belongs to the potassic feldspar family.
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